The UN human rights commissioner has condemned an Egyptian court's decision to sentence to death 528 supporters of ousted President Mohammed Morsi.
A spokesman for Navi Pillay said the "cursory mass trial" was "rife with procedural irregularities" and breached international human rights law.
The defendants were found guilty on Monday of charges relating to an attack on a police station in Minya in August.
Another 683 Morsi supporters went on trial at the same court on Tuesday.
They include the Muslim Brotherhood's general guide, Mohammed Badie, and the chairman of its Freedom and Justice Party (FJP), Saad al-Katatni.
Later, security forces clashed with hundreds of Minya University students protesting against the trials.
Tear gas was fired at the students after they blocked a main road, threw stones and set an armoured police vehicle on fire.
There has been widespread condemnation of Monday's decision by the Minya Criminal Court to sentence 528 people to death for their alleged participation in an attack on a police station in the central city in mid-August, in which a police officer was killed.
The incident took place in the immediate aftermath of an operation by security forces to break up two sit-ins in the capital Cairo that left almost 1,000 people dead. The sit-ins were set up by supporters of Mr Morsi's after he was overthrown by the military the previous month.
The trial, at which more than three-quarters of the defendants were not present, is reported to have lasted less than an hour on Saturday.
The prosecution did not put forward evidence implicating any individual defendant, even though it had compiled significant evidence, and the court prevented defence lawyers from presenting their case or calling witnesses, according to Human Right Watch.
A second session was held on Monday solely to announce the verdict.
On Tuesday, a spokesman for the UN High Commissioner for Human Rights, Rupert Colville, told a news conference in Geneva: "The astounding number of people sentenced to death in this case is unprecedented in recent history."
"The mass imposition of the death penalty after a trial that was rife with procedural irregularities is in breach of international human rights law."
Sixteen Egyptian rights groups voiced concern, saying the trial constituted a "dangerous, unprecedented shift in the Egyptian's judiciary's treatment of such cases" and represented "a grave violation of both the right to a fair trial and the right to life".
But Egypt's interim government defended the court, insisting that the sentences had been handed down only "after careful study".
The state-run al-Ahram newspaper said the court would issue its final verdict on 28 April after Egypt's grand mufti, who under the law must ratify each death sentence before it can be carried out, had passed judgement.
The defendants may then appeal. Legal experts said a higher court would most probably order a retrial or reduce their sentences.
Mr Colville also expressed concern about the 683 people who went on trial at the Minya Criminal Court on Tuesday on similar charges relating to an attack on another police station in which no-one died.
The BBC's Orla Guerin, who is outside the courthouse, says that after an opening session lasting just a few hours the case was adjourned until 28 April, when the judge, Said Youssef, said a verdict would be given.
Critics will ask how the fate of so many people could be decided after the briefest of trials, our correspondent says. And, she adds, there were two key elements missing - most of the defendants and any of their lawyers.
Only 62 defendants reportedly appeared in court, with officials saying Mr Badie and Mr Katatni could not attend for security reasons, while the defence team staged a boycott in protest at the preceding trial.
"As lawyers, we haven't seen anything like what happened here yesterday in our entire professional lives and we will not see anything like it until our deaths," Khaled Fouda of the Minya lawyers' syndicate said.
Despite the boycott, the judge heard testimony from witnesses and questioning several of the defendants before adjourning the case.
The wife of one of the accused in Tuesday's case told the BBC she feared the verdict had been decided in advance. She said her husband was an innocent man who had been arrested while picking up their daughter from hospital.
The 1,200 defendants in the two cases in Minya are among more than 16,000 Egyptians arrested over the past eight months, according to figures recently provided by senior interior ministry officials. They include about 3,000 top or mid-level Brotherhood members.