The Mehr News Agency correspondent at the scene said that the latest reports indicate that at least 25 women and children and 15 men were killed in the incident. Some of the victims died of smoke inhalation.
About 200 people were injured in the fire in the center of the city, according to police and IRIB television.
Reports said the blaze broke out as people gathered in the Ark Mosque near Tehran's main bazaar, according to the BBC.
Fleets of ambulances were sent to the scene to take the injured to nine hospitals in the area and a televised appeal has been made for blood donations.
It is thought that the blaze could have been caused by a heater brought inside the mosque to protect worshippers from the bitter cold.
Reports suggest a tarpaulin roof in the women's section caught fire and the flames soon spread.
Terrified worshippers tried to escape the flames through the main exit, causing a stampede that led to many of the injuries.
"According to our initial estimates there are 35 dead," an Iranian police officer told reporters at the scene.
Some people broke windows and jumped out of the mosque to escape the flames.
MS/HG
End
MNA