Three demonstrators were bloodied by baton-wielding security forces who also arrested at least two people, CNN reported.
Traffic was bumper-to-bumper with car horns sounding. Piles of debris were set afire and tear gas was fired to disperse crowds, witnesses said.
Shiite Muslims, outraged over the June re-election of President Mahmoud Ahmadinejad, rioted in the streets during the holy period of Ashura. Protesters joined Muslims simply partaking in the religious observance, CNN said.
The holiday, the culmination of the first 10 days of the Islamic month Muharram, remembers the doomed seventh century battle of Imam Hussein, a grandson of the prophet Muhammad, to retain the throne of Islam. To commemorate Hussein's struggle, Shiite Muslims ritually whip themselves and chant "Ya Hussein."
Ashura is expected to climax Sunday with a day of mourning for Grand Ayatollah Hossein Ali Montazeri, who was a champion of the opposition protesters, CNN said.