The United States on Tuesday urged Egypt not to execute 529 supporters of deposed Islamist president Mohamed Morsi who were given the death penalty in a mass sentencing which has triggered an international outcry.

Deputy State Department spokeswoman Marie Harf called the sentencing "shocking."

Egypt's military-installed interim president Adly Mansour called Tuesday for Arab ministers to meet before the end of June to discuss implementation of an Arab counter-terrorism treaty.

Mansour made no specific mention of the Muslim Brotherhood of his ousted predecessor Mohammed Morsi, which Cairo designated a terrorist organization in December.

An Egyptian court's sentencing of 529 people to death for rioting has fixed the spotlight on a massive legal war on the Islamist opposition, with thousands in prison and hundreds more convicted in mass trials.

Many of an estimated 15,000 suspects in prison are languishing without charges in overcrowded and filthy cells, rights lawyers say.

Egypt's Muslim Brotherhood leader went on trial Tuesday along with hundreds of other Islamists before the same judge who triggered a global outcry when he sentenced 529 convicted rioters to death.

The trial of Mohamed Badie and around 700 supporters of deposed president Mohammed Morsi was adjourned after just a few hours until April 28, when judgment and sentencing is expected, a lawyer said.

The United States is "deeply concerned" about the death sentences handed out against 529 supporters of Egypt's deposed President Mohammed Morsi, a U.S. official said Monday.

"While appeals are possible, it simply does not seem possible that a fair review of evidence and testimony consistent with international standards could be accomplished with over 529 defendants after a two-day trial," a State Department official said.

Al-Jazeera's Australian reporter Peter Greste, on trial in Egypt with fellow journalists, said in court Monday that he and his colleagues have spent three months in jail on "baseless" charges.

Greste was speaking to reporters from a caged dock as the court heard testimonies from five witnesses, including three security officers.

An Egyptian court sentenced 529 supporters of deposed Islamist president Mohammed Morsi to death on Monday after just two hearings, in the largest mass sentencing in the country's modern history.

The shock verdict, which came amid a sweeping crackdown on Morsi's supporters since his overthrow by the army last July, is likely to be overturned on appeal, legal experts said.

An Egyptian court released a leading left-wing activist on trial for joining an unsanctioned and violent protest on bail Sunday, as the army-installed regime seeks to quell political unrest.

Alaa Abdel Fattah and 24 others are accused of taking part in a violent protest last November outside the senate, where a panel drafted a new constitution giving the army broader powers.

More than 1,200 supporters of Egypt's ousted Islamist president Mohamed Morsi face charges Saturday in the country's biggest trial since an August police crackdown, judicial sources said.

The mass trial is part of a crackdown by Egypt's military-installed authorities targeting Morsi's Islamist supporters who remain steadfast in demanding his reinstatement.

A court in Egypt adjourned the mass trial of more than 1,200 supporters of deposed Islamist president Mohamed Morsi after a brief hearing on Saturday, state media reported.

The country's biggest trial since the army removed Morsi last July after a turbulent single year in office was adjourned until Monday.