TEHRAN: Iranian police fired warning shots in the air and beat opposition protesters in central Tehran Sunday, witnesses and opposition Web sites said.

Authorities had warned of a harsh crackdown should opposition supporters hold rallies coinciding with Sunday's religious observances marking the 7th Century death in battle of one of Shiite Islam's most beloved saints.

Police helicopters circled overhead and black clouds of smoke billowed into the sky over central Tehran. Witnesses said police opened fire with warning shots to try to disperse the crowds.

The pro-reform Web site Rah-e-Sabz said security forces beat protesters as they chanted anti-government slogans.

Police had blocked streets leading to the center of the capital to try to prevent thousands of people from joining the protest. Still, many opposition supporters managed to break the security wall.

Ambulance sirens could be heard near Engelab Square, in central Tehran, where the unrest was taking place.

Opposition activists have held a series of anti-government protests since the death of a dissident cleric last week.

Last Sunday's death of the 87-year-old Grand Ayatollah Hossein Ali Montazeri, a sharp critic of Iran's leaders, has given a new push to opposition protests, which have endured despite a heavy security crackdown since June's disputed presidential election.