Amnesty International says Iranian authorities have executed a man convicted of killing three police officers during clashes involving members of a Sufi order, despite calls to stop his execution.

“We are shocked and saddened to announce that Mohammad [Reza] Salas was executed this morning,” the British-based rights group tweeted on June 18.

During clashes in Tehran in February, Salas drove a bus into a group of police officers during battles between security forces and followers of the Sufi Gonabadi order, known as dervishes.

The dervishes were protesting the arrest of members of the sect, as well as rumors that their 90-year-old leader would soon be detained by police, despite assurances by the authorities that they had no such intention.

During court hearings in March, Salas repeatedly said that he did not intentionally kill police officers, according to local media.

Two members of the paramilitary Basij force were also killed in the skirmishes, authorities said. Some 300 dervishes were reportedly arrested following the violence.

Sufism, a mystical branch of Islam, is not illegal in Iran but rights groups accuse the Iranian government of harassment and discrimination against their followers, including the Gonabadis, one of the largest Sufi sects.