At least nine Iranian demonstrators, including the nephew of opposition leader Mir Hossein Mousavi, were killed on Sunday in dissident clashes with security forces across Iran, according to a pro-reformist Web site.

"Ali Mousavi, 20, was killed in clashes on Sunday noon and his body was still kept in a Tehran hospital," said parlemannews.

An Iranian opposition website said at least four protesters were killed in the northwestern city of Tabriz. Earlier in the day, the Web site said four protesters were killed in a rally in central Tehran while a fifth was reported in the evening.

Iranian police arrested about 300 people during the anti-government protests in Tehran, state broadcaster IRIB quoted a senior police official as saying.

According to the opposition Web site, police forces had begun refusing orders to shoot at pro-reform protesters after the four victims were killed in the Tehran clashes.

"Police forces are refusing their commanders' orders to shoot at demonstrators in central Tehran ... some of them try to shoot into air when pressured by their commanders," the Jaras website said.

Iranian police were firing warning shots in the air and beating opposition protesters in Tehran, other witnesses and opposition Web sites said. Iranian police opened fire on the protesters, Jaras reported.

A senior Iranian police official denied the report that four pro-reform protesters were killed during clashes in Tehran on Sunday, the Students News Agency ISNA reported.

"So far there have been no reports of killings and no one has been killed up to now," Tehran police chief Azizollah Rajabzadeh said, quoted by ISNA.

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.