Three police officers were killed on Monday in clashes with protesters in northern Tehran, a police spokesman said, as tension worsened between Islamic Republic authorities and followers of a Muslim religious order.
Video footage posted on social media on Monday evening showed clashes between security forces and members of a Sufi order in Iran known as the Gonabadi Dervishes. They had gathered in front of a police station, demanding the release of some members of their sect.
"Some disruptors of security and order used a bus today and killed three police officers... The murderers were arrested seconds after their crime," police spokesman Saeed Montazer al-Mahdi said on state television.
A video on social media showed a white bus ploughing through a group of around 40 riot police officers in a narrow street. The bus also strikes several parked cars as it tries to escape the scene. The video's authenticity could not be verified.
Further social media videos showed police firing tear gas to disperse dervishes and other demonstrators.
The Islamic Republic views the Sufi order as a threat to the prevailing Shi'ite establishment and has imprisoned some Gonabadi Dervishes in an ongoing crackdown.
Several dervishes have been arrested in the last two months, according to the Center for Human Rights in Iran (CHRI), a non-profit group based in New York.
Ten members of the Sufi order in Iran were injured and three others arrested in the city of Kowar, in Fars Province after police attacked their rally on Jan. 14, CHRI said. They were also demanding the release of Sufi detainees.