One firefighter, who had managed to escape the 15-storey Plasco building before it fell, died in hospital from severe burns.
One firefighter, who had managed to escape the 15-storey Plasco building before it fell, died in hospital from severe burns, the official IRNA news agency reported.
The building, which was Iran's oldest high-rise and contained a shopping centre and hundreds of clothing suppliers, came down after a four-hour blaze.
Rescue workers, soldiers and sniffer dogs worked through the night in a desperate bid to locate around 20 firefighters who were still inside when it collapsed.
They had so far been unable to reach any survivors or find any bodies, despite the frantic clearing effort and two tunnels being dug under the wreckage of the building.
Iranians were in shock over the apparent loss of so many firefighters, with state television placing a black banner across the corner of the screen as a sign of mourning.
Supreme leader Ayatollah Ali Khamenei said late Thursday that the incident had "caused deep sorrow, regret and concern for me" and he praised the "bravery and sacrifice of the firefighters."
IRNA said 84 people were injured in the fire, of whom five were still in hospital.
"The relief work is very hard. Removal of debris is going on in all parts of the building but the thick smoke rising from the building is creating serious problems," said Tehran emergency services director Pir Hossein Koolivand.
"It is still not clear how many people are trapped under the rubble and not even one person has been pulled out."
President Hassan Rouhani has called for an immediate investigation, with city officials saying the building's managers ignored repeated warnings about fire hazards.
It was built by Habibollah Elghanian, a prominent Iranian-Jewish businessman who was arrested for ties to Israel and sentenced to death and executed after the 1979 Islamic revolution.