CAIRO — Egypt's military assumed responsibility Monday for protecting state institutions and maintaining security ahead of a Dec. 15 constitutional referendum, as the country braced for another round of mass demonstrations by the supporters of the country's Islamist president and the liberal opposition over the disputed charter.

The referendum on a contentious new constitution lies at the heart of a bitter political battle that has deeply polarized Egypt and triggered some of the worst street violence between backers and opponents of President Mohammed Morsi since he took power in June as the country's first democratically elected leader.

So far, Morsi has stood firm on the referendum, refusing to yield to opposition demands that he scrap the vote scheduled for Saturday. The opposition, meanwhile, was still trying to decide late Monday whether to boycott the referendum or rally Egyptians to vote "no" to the draft constitution, and hoping that a massive turnout for a rally Tuesday would force the president to cancel the balloting.

"We still have a chance, with popular rejection, to stop the referendum," said Basil Adel, a former lawmaker and liberal activist.

Egypt's political crisis began on Nov. 22 when Morsi issued a decree granting himself – and the Islamist-dominated panel writing the constitution – immunity from judicial oversight or challenge. Those decrees sparked mass demonstrations, with opponents saying they were issued initially to protect the draft charter from the judiciary.

The constituent assembly then hurriedly approved the draft constitution in a marathon overnight session, further inflaming those who claim that Morsi and his Islamist allies, including the Muslim Brotherhood, are monopolizing power and trying to force their agenda into practice.

That prompted hundreds of thousands of the president's opponents to take to the streets in massive rallies – the largest from primarily secular groups since the uprising that toppled Hosni Mubarak last year. Morsi's supporters responded with huge demonstrations of their own, which led to clashes in the streets that left at least six people dead and hundreds wounded.

Morsi has rescinded the decree that gave him absolute powers, but did not meet the opposition's main demand and delay the referendum.

With tensions running high in the country, the president on Sunday ordered the military to take responsibility for security and protect state institutions along with the police until the results of the constitutional referendum are announced.

On Monday, the army took up the task in line with the presidential decree, which also grants the military the right to arrest civilians.

Presidential spokesman Yasser Ali tried to downplay concerns the move was a step toward martial law, saying instead that "it is merely a measure to extend legal cover for the armed forces while they are used to maintain security."

There were no signs of an increased military presence outside the presidential palace, where tanks have been deployed since last week's fierce street clashes, or elsewhere in the capital on Monday.

Still, the decision to lean on the military, which had a spotty record over the tumultuous 16 months that it ran Egypt after Mubarak's fall, prompted concern among international rights groups.

"Considering the track record of the army while they were in charge, with more than 120 protesters killed and in excess of 12,000 civilians unfairly tried before military courts, this sets a dangerous precedent," said Hassiba Hadj Sahraoui of Amnesty International.

Human Rights Watch called on Morsi to put limits to military's policing powers to prohibit trials of civilians in front of military tribunals.

Activists and rights groups have documented a catalog of human rights violations by authorities during the period of military rule.

Judges have gone on strike to protest Morsi's decrees, which they perceive as an "assault" on the judiciary, and have said they would not oversee the Dec. 15 vote as is customary for judges in Egypt. Judges of the nation's administrative courts announced Monday they were conditionally lifting their boycott of the vote, but they said their supervision of the process was conditional on bringing an end to the siege of the Supreme Constitutional Court by Morsi's supporters.

In exchange for their supervision, they also demanded assurances that authorities would crack down on vote canvassing outside polling stations and offer life insurance policies to the judges.

Against this backdrop, Morsi on Monday suspended a series of tax hikes announced a day earlier on 50 good and services including alcohol, cigarettes and mobile phones.

The decision to raise the taxes just days ahead of a referendum that has deeply divided the nation baffled many in Egypt.

The Muslim Brotherhood's Freedom and Justice Party, which Morsi used to lead, called upon the president to suspend the tax hike until a new parliament is in place to discuss it. Morsi enjoys legislative authorities in the absence of the parliament which was disbanded by a court order.

FJP party member Ashraf Badr Eddin summed up the thinking of many in Egypt: "The timing, politically speaking, is very inappropriate."

Experts say the taxes hikes were part of reform economic package Egypt has to carry in order to secure a badly-needed $4.8 billion loan from the International Monetary Fund.

Egypt reached an initial agreement on Nov. 20 for the loan. The IMF's executive board is expected to vote on the deal in mid-December.

The government views the loan as crucial to revive the country's ailing economy and improve the nation's image in the minds of international donors, which could eventually help Egypt secure more loans to help boost its battered finances.

Economist Ahmed el-Sayyed el-Nagar, from the Al-Ahram Center for Political and Strategic Studies, warned that the flip-flop could hurt Egypt's ability to secure the loan.

"It shows a deficit within the decision-making circles and doubts about the government ability to meet its commitments," he said.

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  • Egyptian protesters chant slogans at rally in Tahrir Square in Cairo, Egypt, Tuesday, Nov. 27, 2012. Egyptians flocked to Cairo's central Tahrir square on Tuesday for a protest against Egypt's president in a significant test of whether the opposition can rally the street behind it in a confrontation aimed at forcing the Islamist leader to rescind decrees that granted him near absolute powers. (AP Photo/ Khalil Hamra)

  • Egyptian protesters attend an opposition rally in Tahrir Square in Cairo, Egypt, Tuesday, Nov. 27, 2012. Thousands flocked to Cairo's central Tahrir square on Tuesday for a protest against Egypt's president in a significant test of whether the opposition can rally the street behind it in a confrontation aimed at forcing the Islamist leader to rescind decrees that granted him near absolute powers.(AP Photo/Khalil Hamra)

  • A masked Egyptian protester takes cover during clashes with security forces near Tahrir square, where an opposition rally has been called for to voice rejection of President Morsi's seizure of near absolute powers, in Cairo, Egypt, Tuesday, Nov. 27, 2012. Egyptian protesters and police clashed in Cairo on Tuesday just hours ahead of a planned massive rally by opponents of the country's Islamist president demanding he rescind decrees that granted him near-absolute powers.(AP Photo/ Khalil Hamra)

  • Tens of thousands people take part in a mass rally against a decree by President Mohamed Morsi granting himself broad powers on November 27, 2012 at Egypt's landmark Tahir Square in Cairo. Clashes between police and protesting youths erupted near Cairo's Tahrir Square, ahead of the demonstration. The planned demonstrations come a day after Morsi stuck by his controversial decree in a meeting with judges that was aimed at defusing the worst political crisis since his election in June. (GIANLUIGI GUERCIA/AFP/Getty Images)

  • An Egyptian protester attempts to throw back a tear gas canister on November 27, 2012 during clashes with the Egyptian Riot Police in Omar Makram street, off Tahrir Square in Cairo. Clashes between police and protesting youths erupted on Tuesday near Cairo's Tahrir Square, ahead of a mass rally against a decree by President Mohamed Morsi granting himself broad powers. The planned demonstrations come a day after Morsi stuck by his controversial decree in a meeting with judges that was aimed at defusing the worst political crisis since his election in June. (GIANLUIGI GUERCIA/AFP/Getty Images)

  • An Egyptian protester blows a stadium horn as he gestures at a cordon of security forces near Tahrir square, where an opposition rally has been called for to voice rejection of President Morsi's seizure of near absolute powers, in Cairo, Egypt, Tuesday, Nov. 27, 2012. Egyptian protesters and police clashed in Cairo on Tuesday just hours ahead of a planned massive rally by opponents of the country’s Islamist president demanding he rescinds decrees that granted him near-absolute powers. (AP Photo/ Khalil Hamra)

  • Egyptian security forces arrest a protester during clashes near Tahrir square in Cairo, Egypt, Tuesday, Nov. 27, 2012. Egyptian protesters and police clashed in Cairo on Tuesday just hours ahead of a planned massive rally by opponents of the country’s Islamist president demanding he rescinds decrees that granted him near-absolute powers.(AP Photo/ Khalil Hamra)

  • An Egyptian protester hurls a stone during clashes with security forces, unseen, in Cairo, Egypt, Wednesday, Nov. 21, 2012. Egyptian protesters firebombed one of the offices of satellite broadcaster Al-Jazeera on Wednesday and attacked a police chief who tried to negotiate an end to three days of violent protests in central Cairo.(AP Photo/Ahmed Gomaa)

  • An Egyptian protester drags a security barrier during clashes outside the country's high court in Cairo, Egypt, Saturday, Nov. 24, 2012. Egypt’s highest body of judges slammed on Saturday a recent decision by the president to grant himself near-absolute power, calling the move an “unprecedented assault” on the judiciary. The statement from the Supreme Judicial Council came as hundreds of demonstrators clashed with police outside a downtown Cairo courthouse. They were protesting the Thursday declaration by President Mohammed Morsi that courts could not overrule his decrees until a new constitution and parliament is in place, several months if not more in the future. (AP Photo/Mohammed Asad)

  • Egyptian protesters gather outside the country's high court in Cairo, Egypt, Saturday, Nov. 24, 2012. Egypt’s highest body of judges slammed on Saturday a recent decision by the president to grant himself near-absolute power, calling the move an “unprecedented assault” on the judiciary. The statement from the Supreme Judicial Council came as hundreds of demonstrators clashed with police outside a downtown Cairo courthouse. They were protesting the Thursday declaration by President Mohammed Morsi that courts could not overrule his decrees until a new constitution and parliament is in place, several months if not more in the future. (AP Photo/Mohammed Asad)

  • Egyptian protesters gather in Tahrir Square in Cairo, Egypt, Sunday, Nov. 25, 2012. President Mohammed Morsi edicts, which were announced on Thursday, place him above oversight of any kind, including that of the courts. The move has thrown Egypt's already troubled transition to democracy into further turmoil, sparking angry protests across the country to demand the decrees be immediately rescinded. The banner in Arabic, top center, reads, "members of the Muslim Brotherhood are not allowed." (AP Photo/Ahmed Gomaa)

  • An Egyptian protester shouts political slogans against President Mohamed Morsi's decree granting himself broad powers as others wave their national flag during a demonstration in Cairo's Tahrir Square on November 27, 2012. The planned demonstrations come a day after Morsi met with the country's top judges in a bid to defuse the crisis over the decree, that has sparked deadly clashes and prompted judges and journalists to call for strike. (GIANLUIGI GUERCIA/AFP/Getty Images)

  • An Egyptian protester recovers from tear gas inhalation on November 27, 2012, during clashes with the Egyptian Riot Police in Omar Makram street, off Tahrir Square in Cairo. Clashes between police and protesting youths erupted near Cairo's Tahrir Square, ahead of a mass rally against a decree by President Mohamed Morsi granting himself broad powers. The planned demonstrations come a day after Morsi stuck by his controversial decree in a meeting with judges that was aimed at defusing the worst political crisis since his election in June. (GIANLUIGI GUERCIA/AFP/Getty Images)

  • A group of protesters shout slogans against Egypt's President Mohammed Morsi during a demonstration in front of the White House in Washington on November 26, 2012. Morsi stuck by a controversial decree granting him sweeping powers, on the eve of planned nationwide rallies to protest the move, in the worst crisis since his election in June. (JEWEL SAMAD/AFP/Getty Images)

  • A group of protesters shout slogans against Egypt's President Mohammed Morsi during a demonstration in front of the White House in Washington on November 26, 2012. Morsi stuck by a controversial decree granting him sweeping powers, on the eve of planned nationwide rallies to protest the move, in the worst crisis since his election in June. (JEWEL SAMAD/AFP/Getty Images)