TORONTO — Police officers and militia forces clashed with demonstrators in central Tehran all day Saturday and then again in northern Tehran in the evening, where the government forces shut down a speech by former President Mohammad Khatami, a reformist leader.

Iranians flagellated themselves on Saturday in Tehran for Ashura, a day of Shiite mourning that erupted in protests.

The demonstrators, who defied an official ban and turned a Shiite ceremony into a protest, underlined the government’s inability to suppress the opposition despite the use of violence. Protests have continued since a disputed presidential election in June, and a large one is anticipated on Sunday, the culmination of the Shiite holiday of Ashura.

Witnesses and an...