* Mass trials could trigger more violence in Egypt
* Brotherhood believed to have about one million members
* United States, EU condemn Monday's mass death sentence (Adds defence lawyer's comment, EU, changes dateline)
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, their lawyer said, in a further blow to supporters of deposed Islamist president Mohamed Mursi.
The trial of Brotherhood Supreme Guide Mohamed Badie, 70, and the others came a day after the same court in Minya Province condemned 529 members of the Islamist group to death, in what rights groups said was the biggest mass death sentence handed out in Egypt's modern history.
The Brotherhood has faced a severe crackdown since the army toppled the group's Mursi - Egypt's first freely elected president - last July following mass protests against his rule.
Defence lawyers boycotted Tuesday's court session - attended by 60 of the defendants - after complaining of irregularities.
"We refrained from attending... because the judge has violated criminal law procedures and did not allow the (lawyers) to present their defence," Adel Ali, a member of the defence team, told Reuters.
He said that 77 of the defendants were in custody while the rest had either been released on bail or were on the run.
The mass trials could ratchet up tensions and trigger more violence in the biggest Arab state, which has been dogged by political upheaval since a popular uprising backed by the army toppled veteran leader Hosni Mubarak in 2011.
Last August, security forces killed hundreds of Brotherhood supporters who staged a long sit-in to demand Mursi's reinstatement. Thousands of others were arrested and top leaders, including Mursi himself, are also on trial.
Islamist militants based in the Sinai near Israel's border have stepped up attacks against police and soldiers since Mursi was ousted by army chief Field Marshal Abdel Fattah al-Sisi.
Hundreds have been killed and the Islamist insurgency is spreading across the country, with shootings, suicide bombings and assassinations of senior Interior Ministry officials.
The Brotherhood, believed to number about one million in a population of 85 million, has won most of the elections held since Mubarak's fall.
The government has declared the Brotherhood a "terrorist" group. The group says it is committed to peaceful activism.
Although the Brotherhood has been squeezed hard by the state, history suggests it will survive the crackdown. The group has endured state repression for decades.
Human rights groups said Monday's verdict suggested the authorities intended to increase pressure on the opposition. The U.S. State Department said it was shocked by the death sentences.
"It defies logic that over 529 defendants could be tried in a two-day period in accordance with international standards," State Department spokeswoman Marie Harf said.
European Union High Representative Catherine Ashton also criticised the court's verdict and called for the authorities to guarantee a fair trial.
"This is particularly important for the credibility of Egypt's transition towards democracy," she said in a statement. (Additional reporting by Asma Alsharif and Yasmine Saleh; Writing by Michael Georgy; Editing by Gareth Jones)