BEIRUT, Lebanon  Police officers in Iran opened fire into crowds of protesters on Sunday, killing at least 10 people, witnesses and opposition Web sites said, in a day of chaotic street battles that threatened to deepen the country’s civil unrest.

The protests, during the holiday commemorating the death of Imam Hussein, Shiite Islam’s holiest martyr, were the bloodiest and among the largest since the uprisings that followed the disputed presidential election last June, witnesses said. Hundreds of people were reported wounded in cities across the country, and the Tehran police said they had made 300 arrests.

One of the dead was Ali Moussavi, a 43-year-old nephew of the opposition leader Mir Hussein Moussavi.

The decision by the authorities to use deadly force on the Ashura holiday infuriated many Iranians, and some said the violence appeared to galvanize more traditional religious people who have not been part of the protests so far. Historically, Iranian rulers have honored Ashura’s prohibition of violence, even during wartime.