Sectarian violence has left at least two dead and 40 others injured in Afghanistan.
Hundreds of Shia Muslims clashed with Sunni Muslims in the western Afghan city of Herat during an important Shia festival.
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.
The fighting started after a gang of about 300 Sunnis threw stones at a Shia 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 Shia responded by attacking Sunnis in a refugee camp, and the violence spread across Herat, with both sides throwing grenades at each other, burning about a dozen cars and two mosques.
Police fired into the air but failed to separate the two sides.
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, Sayar said.
Sunnis make up about 80 percent of Afghanistan's 28 million people, and Shias 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 US-backed government and foreign forces.
AP