Ahead of a planned opposition rally on Monday, Iran tightened security and arrested over 20 mothers who were mourning children killed in the unrest that has broken out since the disputed June 12 elections.

The mothers took part in an antigovernment protest in Leleh Park in central Tehran every Saturday since the death in June of Neda Agha-Soltan, 26, whose shooting became a symbol of the government’s violent repression. The rally had been attacked by the police before, but Saturday was the first time the mothers were arrested.

An opposition Web site reported that the protest was broken up by the police and many demonstrators were taken away. The BBC Persian service quoted a witness who said 29 women were arrested, some of whom were later released. But at least 21 remained in jail, the BBC said.

Ms. Agha-Soltan’s mother regularly attended the rally, but it was not clear whether she was there on Saturday or was among those arrested.