Washington and the European Union expressed concern and questioned the fairness of proceedings against so many defendants lasting just two days.
But Egypt's army-installed interim government defended the court's handling of the case, insisting that the sentences had been handed down only "after careful study" and were subject to appeal.
The defendants who were sentenced to death are part of a larger group of more than 1,200 alleged Islamists accused of killing two policemen and rioting on August 14, after police killed hundreds of protesters while dispersing two Cairo protest camps.
Of the 529, only 153 are in custody. The rest were tried in their absence and have the right to a retrial if they turn themselves in.
Another 17 defendants were acquitted.
The judgement can be appealed at the Court of Cassation, which would probably order a new trial or reduce the sentences, legal expert Gamal Eid said.
"This sentencing is a catastrophe and a travesty and a scandal that will affect Egypt for many years," said Eid, who heads the Arabic Network for Human Rights Information.
- Speed of trial criticised -
Washington questioned how the court could have given the defendants a fair hearing in a trial that spanned just two days -- an opening session on Saturday and Monday's sentencing.
"It sort of defies logic," said Marie Harf, deputy US State Department spokeswoman, adding that Washington was "deeply concerned" and "shocked".
"Obviously the defendants can appeal, but 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 in a two-day trial," Harf said.
EU foreign policy chief Catherine Ashton said "capital punishment can never be justified" and urged Egyptian authorities to grant defendants "the right to a fair and timely trial".
Defence counsel Mohamed Tousson charged that the judge had rushed to sentencing on Monday after being angered by a lawyer's request for his recusal at Saturday's opening hearing.
"He got very angry, and adjourned the trial for sentencing," Tousson said. "It's a huge violation of defendants' rights."
The foreign ministry defended the court's handling of the trial, saying that the sentences had been "issued by an independent court after careful study of the case".
It said the Egyptian judiciary was "entirely independent and is not influenced in any way by the executive branch of government".
Amnesty International said it was the "largest single batch of simultaneous death sentences we've seen in recent years, not just in Egypt but anywhere in the world," and called for the verdicts to be quashed.
A second group of about 700 defendants, including Mohamed Badie, the supreme guide of Morsi's Muslim Brotherhood movement, is due in the dock on Tuesday.
The Muslim Brotherhood said the death sentences were yet "another indication that the corrupt judiciary is being used by the coup commanders to suppress the Egyptian revolution and install a brutal regime."
At least 1,400 people have been killed in the crackdown on Morsi's supporters and thousands more arrested, according to Amnesty International.
Morsi is himself currently on trial in three different cases. The army removed Egypt's first freely elected president after a single year in power following mass protests demanding his resignation.
- 'Baseless charges' -
The ensuing crackdown has also targeted prominent activists of the 2011 uprising against veteran president Hosni Mubarak, as well as journalists.
A group of journalists working for Al-Jazeera television was back in court on Monday for the third hearing in their trial on charges of spreading false news and aiding the blacklisted Brotherhood.
Award-winning Australian reporter Peter Greste told reporters from the caged dock that he and his colleagues were being held on "baseless charges".
"We haven't seen any evidence in the court that possibly justify the charges or our imprisonment," said Greste.
"We spent three months in prison based on baseless charges."
Journalists around the world have held protests demanding the reporters' release, with the United Nations and the United States also voicing their support.
Egypt's interim president Adly Mansour has pledged to work towards a "speedy resolution of the case".