CAIRO -- Mob-led sexual assaults targeting Egypt's female protesters could increase if perpetrators are not punished, an international rights group warned on Wednesday.

Amnesty International said statements from victims show that the assaults follow a "clear pattern," where mobs of men encircle the victims, assault them with weapons and hands and then try to undress them.

Amnesty's warning followed a statement from the U.N. human rights office, which last week said that about 25 women were reportedly sexually assaulted – in some cases with extraordinary violence – in Cairo's Tahrir Square during recent demonstrations against Egypt's Islamist President Mohammed Morsi.

The square was the center of the 2011 uprising that toppled former President Hosni Mubarak. It has been the scene of a number of assaults against women – both protesters and journalists – in the aftermath of the uprising and a place where women have been stripped, groped and raped at demonstrations.

The U.N. agency demanded that Egyptian authorities take steps to bring the perpetrators to justice. Amnesty also urged prompt action.

"Horrific, violent attacks on women, including rape in the vicinity of Tahrir Square, demonstrate that it's now crucial President Morsi takes drastic steps to end this culture of impunity and gender-based discrimination," said Hassiba Hadj Sahraoui of the London-based group.

Amnesty cited a report by a local anti-harassment group, which said that a total of 19 violent attacks against women took place on Jan. 25 alone – the day Egyptians staged a huge demonstration in Cairo to mark the second anniversary of the uprising that ousted Mubarak.

Activists have called the incidents the worst in years, describing them as the darkest stain on the country's opposition street movement.

One particular attack on a woman on Jan. 25 has stood out: A mob of men on Cairo's Tahrir Square raped a 19-year-old woman with a sharp object, cutting her genitals in an attack that forced her to undergo emergency surgery.

Also Wednesday, some 2,000 people rallied in downtown Cairo, chanting against Morsi's Islamist government for not doing anything to protect women demonstrators. Some at the rally raised banners reading: "Those silent against the harassers are devils."

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  • An Egyptian shouts slogans during anti-President Mohammed Morsi protest in front of the presidential palace in Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Amr Nabil)

  • Egyptians shout slogans during an anti-President Mohammed Morsi protest in front of the presidential palace in Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Amr Nabil)

  • Egyptians shout slogans during anti-President Mohammed Morsi protest in front of the presidential palace in Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Amr Nabil)

  • Egyptians shout slogans during an anti-President Mohammed Morsi protest in front of the presidential palace in Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Amr Nabil)

  • Egyptian protesters shout anti-Mohammed Morsi slogans under a giant flag in front of the presidential palace in Cairo, Egypt, Friday, Feb. 1, 2013. Arabic banner reads, "Hamas, brotherhood, Qatar and America, are Egyptian enemies." (AP Photo/Amr Nabil)

  • Egyptian protesters chant anti- government slogans during a rally in Tahrir Square, Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Khalil Hamra)

  • Egyptian President Mohammed Morsi, center, waves after attending Friday prayers in Cairo, Egypt, Feb. 1, 2013. (AP Photo/Jihan Nasr, Shorouk Newspaper)

  • Egyptian protesters chant anti- government slogans during a rally in Tahrir Square, Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Khalil Hamra)

  • An Egyptian masked protester wears a head banner with Arabic writing that reads "Egypt", during a rally in Tahrir Square, Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Khalil Hamra)

  • An Egyptian street vendor sells vegetables on a motorbike during a rally in Tahrir Square, Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Khalil Hamra)

  • An Egyptian protester tries to escape from fire after he burned an anti-Mohammed Morsi banner in front of the presidential palace in Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Amr Nabil)

  • Egyptians wave a giant flag during anti-President Mohammed Morsi protest in front of the presidential palace in Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Amr Nabil)

  • Egyptian security forces protect themselves from heavy rains in front of the presidential palace in Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Amr Nabil)

  • Egyptian protesters chant anti--government slogans during a rally in Tahrir Square, Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Khalil Hamra)

  • Egyptian protesters chant anti-government slogans during a rally in Tahrir Square, Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Khalil Hamra)

  • Egyptian protesters chant anti-government slogans during a rally in Tahrir Square, Cairo, Egypt, Friday, Feb. 1, 2013. (AP Photo/Khalil Hamra)