Five Iranian security personnel have been killed in clashes with members of a Sufi Muslim group, police say.
Three policemen were killed by a bus, a security official was stabbed to death and a fifth killed in a car-ramming attack, police said.
It happened as Gonabadi dervishes protested over the detention of one of their members in northern Tehran.
Rights groups say Sufi dervishes are persecuted by the Iranian authorities, who see their practices as a threat.
They regard the religious order as going against the Shiism endorsed by the state.
Often described as Islamic mysticism, Sufism emphasises the inner search for God and the renunciation of worldly matters.
More than 300 protesters, and the driver of the vehicles, were arrested in connection with the violence, police said.
BBC Persian posted a video of the incident on their Twitter account.
لحظهای که یک #اتوبوس ماموران پلیس را زیر میگیرد.— BBC Persian (@bbcpersian) February 19, 2018
هشدار: حاوی تصاویر آزاردهنده
سخنگوی پلیس میگوید در جریان این حمله در محدوده درگیری #دراویش و ماموران ویژه در محله پاسداران تهران “سه مامور کشته شدهاند”. pic.twitter.com/xJe9KFC5zP
Videos published on social media before the clashes showed dervishes holding a sit-in protest in front of a police station where one of their members was being held.
A video posted on Twitter shows some protesters marching in the middle of the road and some others sitting on the road, holding banners with the photo of the arrested dervish, chanting "God is great."
Dervishes reported that the police used tear gas to disperse their gathering and videos on social media showed riot police chasing the protesters.