Hundreds of Shiite Muslims and Sunnis clashed in a western Afghan city Thursday during an important Shiite festival, hurling grenades and burning mosques, officials and witnesses said. At least four people were killed and 51 injured.

Islamic extremists are suspected to have incited the violence, said Ismatullah Mohammed, a senior police officer.

The fighting followed three days of rioting across Afghanistan over drawings of the Prophet Muhammad, first published in a Danish newspaper. Those riots left 11 dead.

Thursday's fighting started after about 300 Sunnis threw stones at a Shiite mosque during the sect's most important festival, Ashoura, Mohammed said. Such an attack is rare in Afghanistan, where there has been little tension between the two sects.

The Shiites responded by attacking Sunnis in a camp for refugees from Afghanistan's past wars, and the violence spread across the city of Herat, with both sides throwing grenades at each other, burning about a dozen cars and two mosques, he said.

Police fired into the air but failed to separate the two sides, said local resident Abdul Nafai.

The deputy army commander in Herat, Faizil Ahmad Sayar, said at least two people had been killed. Barakatullah Mohammedi, a doctor at Herat Hospital, said at least four people died and 51 were injured.

Mohammed said hundreds of young men were believed to be coming into Herat from surrounding towns and villages to join the fighting and security forces have been ordered to block the main roads.

Thousands of other soldiers and police have fanned out across the city, according to the deputy army commander in Herat, Faizil Ahmad Sayar.

Sunnis make up about 80 percent of Afghanistan's 28 million people, and Shiites 20 percent. Apart from a small clash in the capital, Kabul, during Ashoura last year, there has been little fighting between the two sects.

Taliban rebels, who are Sunnis, have directed their campaign of violence at the country's U.S.-backed government and foreign forces, not the Shiites.