TEHRAN, Iran — Iranian police and plainclothes security agents broke up a sit-in marking yesterday’s anniversary of a bloody raid on a Tehran university dormitory, then stormed the offices of the country’s main pro-democracy student group, student leaders said. Fifteen students and a mother were beaten and detained, they said.

Iran had banned street protests to mark the anniversary of July 9, 1999, raid by police and hard-line vigilantes on a Tehran University dormitory that killed one person and injured at least 20. The attacks triggered six days of nationwide protests, the worst since the 1979 Islamic revolution that toppled the Shah and brought hardline clerics to power.

Pouya Ifaei, a student leader, said the students organized their sit-in at Amir Kabir University yesterday to protest the continued detention of eight students been in custody since May on vague charges and to mark the anniversary of the 1999 attack.