* Mass trials, death sentences could trigger more violence
* Brotherhood believed to have about 1 million members
* United States, EU condemn Monday's mass death sentence (Adds call for protests)
By Treza Kamal
MINYA, Egypt, March 25 (Reuters) - The leader of Egypt's outlawed Muslim Brotherhood and 682 others went on trial on Tuesday on charges including murder, a day after more than 500 supporters of deposed Islamist president Mohamed Morsi were sentenced to death.
The mass trials and death sentences - the biggest in Egypt's modern history - are signs that the crackdown on Morsi's Brotherhood is intensifying ahead of presidential elections that army chief Field Marshal Abdel Fattah al-Sisi is expected to win.
The Brotherhood's Supreme Guide Mohamed Badie, 70, and the others are being tried in the same court that one day earlier condemned 529 members of the Islamist group to death , a verdict condemned by foreign governments and rights groups.
Defense lawyers boycotted Tuesday's court session after complaining of irregularities and reporters were barred.
"We refrained from attending ... because the judge has violated criminal law procedures and did not allow the (lawyers) to present their defense," Adel Ali, a member of the defense team, told Reuters.
Protests erupted after Tuesday's trial began, with police firing teargas to deter hundreds of demonstrators. An Islamist alliance that includes the Brotherhood has called on Egyptians to stage nationwide protests on Wednesday.
The U.N. human rights office said Monday's mass death sentences contravened international law and the ruling was also criticized by the European Union and the United States.
"Yesterday was ... a death sentence for the credibility and independence of Egypt's criminal justice system," said Nicholas Piachaud of Amnesty International.
"There is little hope of the 683 people indicted in this latest trial of receiving fair proceedings before the same judge who yesterday handed down death sentences so readily."
There are no signs that Western powers will back their dismay with action to push for greater democracy in Egypt, which is of strategic importance because of its peace treaty with Israel and for the Suez Canal, a major global shipping lane.
Justice Ministry official Abdel Atheem al-Ashari defended the death sentences, saying in a response to the ruling that the separation between the state and the judiciary was a principle of any democratic system.
Egypt has cracked down hard on the Brotherhood since Sisi toppled Morsi, Egypt's first freely elected president, in July, and installed a government.
In August, security forces killed hundreds of Brotherhood supporters who were staging a sit-in to demand Morsi's reinstatement. Thousands of others were arrested and leaders, including Morsi himself, are on trial.
Seventy-seven of those on trial on Tuesday are in custody while the rest have been released on bail or are on the run, defense lawyer Ali said. The verdicts are due on April 28.
All the charges relate to clashes in Minya, a bastion of Islamist support south of Cairo, that broke out after the security forces crushed the pro-Morsi camps in the capital. A policeman was killed during the protests.
Hours after Tuesday's trial began, protests broke out at Minya University. Police lobbed teargas canisters and fired in the air in an attempt to disperse hundreds of demonstrators.
In Egypt's second city Alexandria, a Reuters witness said protesters chanting against Sisi marched out of the main gate of a university and blocked a busy road. Some raised their hands to display the four-finger sign that has become a symbol of sympathy for the Brotherhood.
Security forces fired teargas, birdshot and live rounds into the air, while protesters threw stones.
"We're coming out (to protest) today because the judiciary has become a tool in the hands of the military and the authorities," said Mohamed Ashraf, a student in the faculty of commerce. "This is evidence of a military coup in Egypt."
The pro-Morsi Islamist alliance has urged Egyptians to take to the streets in politically sensitive areas of Cairo on Wednesday despite severe restrictions on demonstrations.
The sites include the Rabaa al-Adawiya mosque, scene of one of two pro-Morsi protest camps and Tahrir Square, the symbolic heart of the revolt against Mubarak.
The Brotherhood, believed to number about 1 million in a population of 85 million, has been declared a terrorist group by the government which blames it and other Islamist groups for attacks on police and soldiers since Morsi was deposed. The Brotherhood says it is committed to peaceful activism. (Additional reporting by Asma Alsharif and Yasmine Saleh; Writing by Michael Georgy; Editing by Alison Williams and Robin Pomeroy)