The death sentences against former president Mohamed Morsi and more than 100 others is "not in line with the Egyptian's obligations under the international law," the European Union said on Sunday.
"The Egyptian judicial authorities have to ensure, in line with international standards, the defendants' rights to a fair trial and proper and independent investigations," EU foreign police chief Federica Mogherini said in a statement.
On Saturday, Cairo Criminal Court handed a preliminary death sentence to Morsi and 105 other defendants on charges related to breaking out of jail during the 2011 unrest.
The court also issued the same ruling to Muslim Brotherhood leaders Mohamed El-Beltagy and Khairat El-Shater, along with 14 others, in another case on charges of conspiring with foreign powers to destabilise Egypt.
The EU said it was confident the sentence will be revised in the appeal phase and also reiterated its opposition to capital punishment.
"The death penalty is cruel and inhumane, fails to act as a deterrent and represents an unacceptable denial of human dignity and integrity," Mogherini said.
The verdicts brought about criticism from several international parties.
A State Department official, who spoke to Reuters on condition of anonymity, said the US was "deeply concerned" about the court decision, while Turkish President Recep Tayyip Erdogan said it was like a return to "ancient Egypt."
Commenting on the wave of criticism that followed the verdicts, Egypt's state official public relations agency said critics reflected "ignorance and inaccuracy."
The statement said the appeal process will go ahead after the Grand Mufti's advisory opinion and once the final verdict is issued on 2 June. It also called on critics "not to forget the nature of the charges levelled against the defendants".