The United States is "deeply troubled" by an Egypt court's death sentence for the Muslim Brotherhood leader Mohammed Badie and 682 supporters, the White House has said.
The same court also reversed 492 death sentences out of 529 it passed in March, commuting most of those to life in prison.
The defendants sentenced were today accused of involvement in the murder and attempted murder of policemen in the southern Minya province on 14 August.
Police killed hundreds of ousted Islamist president Mohammed Mursi's supporters in clashes in Cairo the same day.
In a statement, the White House said: "Today's verdict, like the one last month, defies even the most basic standards of international justice.
"The Egyptian government has the responsibility to ensure that every citizen is afforded due process, including the right to a fair trial in which evidence is clearly presented, and access to an attorney.
"We urge the Egyptian government to end the use of mass trials, reverse this and previous mass sentences, and ensure that every citizen is afforded due process."
UN Secretary General Ban Ki-moon is "alarmed" by the Egyptian court's decision and fears it could impact the entire region, his spokesman said.
"Verdicts that clearly appear not to meet basic fair trial standards, particularly those which impose the death penalty, are likely to undermine prospects for long-term stability," Mr Ban said.
Several female relatives waiting outside the courtroom fainted on hearing news of the verdict.
Seeking the death penalty for Mr Badie, the Brotherhood's general guide, is certain to raise tension in Egypt.
The country has been gripped by turmoil since the army removed the Brotherhood from power last year.