Islamist supporters of President Mohamed Morsi captured, detained and beat dozens of his political opponents last week, holding them for hours with their hands bound on the pavement outside the presidential palace while pressuring them to confess that they had accepted money to use violence in protests against him.

“It was torment for us,” said Yehia Negm, 42, a former diplomat with a badly bruised face and rope marks on his wrists. He said he was among a group of about 50, including four minors, who were held on the pavement overnight. …

The abuses, during a night of street fighting between Islamists and their opponents, have become clear through an accumulation of video and victim testimonies that are now hurting the credibility of Mr. Morsi and his allies as they push forward to this weekend’s referendum on an Islamist-backed draft constitution. To critics of Islamists, the episode on Wednesday recalled the tactics of the ousted president, Hosni Mubarak, who often saw a conspiracy of “hidden hands” behind his domestic opposition and deployed plainclothes thugs acting outside the law to punish those who challenged him. The difference is that the current enforcers are driven by the self-righteousness of their religious ideology, rather than money.