The convictions on Sunday are "of great concern", Ravina Shamdasani said, calling for the "immediate and unconditional release of all prisoners detained solely in connection with peaceful protests, unless the authorities have solid evidence (of) recognisable criminal offences."
The convictions of Ahmed Maher, Ahmed Douma and Mohamed Adel were the first against pro-democracy protesters -- as opposed to Islamists -- since the July 3 overthrow of president Mohamed Morsi, whose Islamist supporters have borne the brunt of a deadly crackdown.
The statement said "dozens of individuals, including students, have been arbitrarily detained and some convicted following what appears to be the exercise of their legitimate rights to peaceful assembly and expression" in Egypt.
UN Secretary-General Ban Ki-moon also joined the chorus of criticism, saying via spokesman Martin Nesirky that the convictions were "contrary to the spirit of Egypt's revolution nearly three years ago."
Ban also "reminds Egyptian officials that freedom of assembly and expression are essential elements for credible elections," Nesirky added.