Around 60 people have died and 200 more were injured after a faulty electrical heater started a blaze in a Tehran mosque crowded with worshippers for a major Shia Muslim festival, Iranian state television reported.
A reporter driving to the mosque said roads had been blocked and ambulances were rushing from the scene on Monday.
The students news agency ISNA added that an explosion was heard and that tents inside the mosque caught fire.
Terrified worshippers trampled others trying to escape, some smashing windows in their desperation to escape the flames.
Shia Muslims are pouring into mosques to commemorate the death of the seventh century Shia martyr Husain, who was killed on the battlefield of Karbala in Iraq.
Tents are common props in plays commemorating Husain's last days.