TEHRAN, Iran - Iranian security forces clashed with antigovernment protesters yesterday in northern Tehran following a speech there by opposition leader and former president Mohammad Khatami, one day before a scheduled larger protest at the climax of a major Shiite religious commemoration.

After the police intervention, thousands of protesters fanned out through the area. The roads were clogged with cars, many honking their horns in apparent support, and about 50 people attacked a building used as an office by the household of the late Ayatollah Khomeini, founder of the Islamic republic, according to the parlemannews Web site, which is critical of the government.

Khomeini's children, who are known supporters of the opposition, were present as the windows were smashed. "Abolfalz, the standard-bearer, keep Khamenei safe!" the attackers shouted, referring to a Shiite saint and Supreme Leader Ayatollah Ali Khamenei, the Web site reported.

Earlier, dozens of members of the Revolutionary Guard Corps and the paramilitary Basij force clashed with small groups of protesters along Revolution Street, on one of the capital's main thoroughfares, at times beating people, urging them to disperse. Some protesters shouted slogans in support of the political leader of the opposition, Mir Hossein Mousavi.

The protests, which followed antigovernment demonstrations in three Iranian cities earlier this week, come as Iran marks the 10 days of Muharram, a mourning period for Imam Hussein, the Shiite saint whose death in the seventh century sealed the rift between Sunni and Shiite Muslims over the succession of the prophet Muhammad.

Today, Shiites worldwide commemorate the day of his death during Ashura, which means "10th" in Arabic.

This year, Ashura falls on the seventh day since the death of the opposition movement's religious leader, Grand Ayatollah Hossein Ali Montazeri. Messages posted on the Internet and spread through Farsi-language opposition satellite channels have called for a large demonstration today in the center of Tehran, but opposition leaders released no statements about such a gathering.

The violence used by security forces yesterday suggested that protests today could be met with strong resistance.

On the streets, protesters mostly fled the security forces. A young man in a gray sweatshirt jumped over a steel fence marking the street, but helmeted, black-clad officers struck him down with batons and dragged him back by an arm and an ear. Another man tried to stop officers from beating a woman. "Let her go!" he yelled, as cars honked in protest.

According to the official student news agency ISNA, the commander of the national police force, Gen. Esmail Ahmadi Moghaddam, said yesterday, "The police have used minimum violence in countering political unrest, but when these unrests reach the point of causing destruction and chaos, we counter it harshly."