Iranian officials say at least five members of the country’s security forces were killed and more than 300 people arrested during clashes between Sufi protesters and police in the capital, Tehran.
Broadcast video appeared to show the moment a bus ploughed into a group of police officers, killing three of them.
Members of Iran's Gonabadi Sufi order, known as dervishes, were protesting the arrest of members of the sect.
Sufism is a mystical strain of Islam characterized by an inward search for God and the rejection of materialism.
There are no official statistics on the size of the Sufi population in Iran, which is subject to continued scrutiny and harassment because the Iranian government views them as heretics.