LONDON (Reuters) - Three police officers were killed in Tehran on Monday, a police spokesman said, as clashes broke out with members of a Muslim religious order seen as a threat to the Shi’ite theocratic establishment.

Video footage posted on social media on Monday evening showed clashes between security forces and members of the Gonabadi Dervishes, an order following the mystical Sufi strain of Islam. 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.

Fars news agency reported that the driver was a member of the Sufi sect. It also said several police officers and pedestrians had been injured.

Further social media videos showed police firing tear gas to disperse Dervishes and other demonstrators. There were also photos of injured demonstrators, with faces covered in blood.

According to Majzooban, a website linked to the Gonabadi Sufis, some protesters were shot by the police. The website also reported that some injured protesters had been arrested in hospitals.

Reuters could not independently verify the reports.

According to the Center for Human Rights in Iran (CHRI), a non-profit group based in New York, several Dervishes have been arrested in the last two months.

It said 10 members of the order were injured and three others arrested in the city of Kowar, in Fars province, on Jan. 14 after police attacked a rally demanding the release of other Sufi detainees.

The late U.N. Special Rapporteur on Human Rights in Iran voiced concern in 2017 about the “targeting and harsh treatment” of members of various Sufi groups, including the Gonabadi order and the Yarsan, also known Ahl-e Haqq.

Asma Jahangir, who died a week ago, said in her report to the U.N. Human Rights Council last March that the groups “continue to face arbitrary arrest, harassment and detention, and are often accused of national security crimes such as acting against national security”.