"The astounding number of people sentenced to death in this case is unprecedented in recent history," said Rupert Colville, spokesman for the UN high commissioner for human rights.
"The mass imposition of the death penalty after a trial that was rife with procedural irregularities is in breach of international human rights law."
The sentences were handed down on Monday after a trial that lasted just two days, sparking an international outcry and sending a chill through opponents of Egypt's military-installed regime, which has placed more than 2,000 alleged Islamists on mass trials since the army overthrew Morsi in July.
"A death sentence may only be imposed after proceedings that meet the highest level of respect for fair trial and due process standards," Colville told reporters.
"A mass trial of 529 people conducted over just two days cannot possibly have met even the most basic requirements for a fair trial," he said.
"The exact charges against each defendant are unclear as they were not read out in court."
Lawyers said they had insufficient access to the defendants and that the court did not consider relevant evidence presented by the defence, he said.
Sources at the trial spotlighted other procedural irregularities including the judge's failure to call on each defendant by name or verify their legal representation, and not bringing some 50 of the defendants to court even though they were in custody.
The defendants were convicted of various charges, including membership of an unlawful organization, Morsi's Muslim Brotherhood movement, incitement to violence, vandalism, unlawful gathering and the killing of one police officer.
Under international rights law, the death penalty may be imposed only for the "most serious crimes", like murder or intentional killing and "membership of a political group or participation in demonstrations certainly do not meet the threshold of 'most serious crimes,'" Colville said.
The same court that had handed down the sentences, in the southern province of Minya, on Tuesday began the trial of 700 more alleged supporters of Morsi, including the leader of the Muslim Brotherhood.