Iranian state TV said at least 78 people, including 45 firefighters, had been hurt when the Plasco building in the south of the capital came crashing down in a giant cloud of dust. One witness described it as "like a horror movie".

Mayor Mohammad Baqer Qalibaf denied speculation on social media that dozens of people had been killed. "The reports on 30, 50 fatalities are incorrect. Around 25 firefighters are trapped inside and rescue teams are trying to take them out," he told state television.

The broadcaster reported: "Still some parts of the collapsed building are on fire. Firefighters are trying to control the fire." Most of those hurt had been taken to hospital and many were quickly discharged, it said.

The semi-official Tasnim news agency said troops had been sent to help dig through the ruins. It said one of the first firefighters to be reached had demanded to be let back inside to save his colleagues.

The agency quoted an official in the Tehran governor's office as saying an electrical short-circuit had caused the fire, but there was no immediate confirmation of this.

President Hassan Rouhani ordered an immediate investigation and compensation for those affected.

RESCUE COULD TAKE DAYS

Tehran Fire Department spokesman Jalal Maleki said the building had collapsed vertically. "That is why adjacent buildings were not damaged," he said.

Occupants of the building had been evacuated as the firefighters tackled the blaze. State TV said the tenants included garment manufacturers, and broadcast footage of business owners trying to re-enter the wreckage.

Sniffer dogs searched for signs of survivors buried under giant slabs of concrete and heaps of twisted metal. The rescue operation could last more than two days, state TV said.

The Plasco building, Iran's first private high-rise, was built more than 50 years ago by a prominent Iranian-Jewish businessman who was arrested and sentenced to death for ties to Israel after the 1979 Islamic revolution.
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Tasnim said it "had caught fire in the past". A fire department spokesman told state TV that tenants "had been warned repeatedly in the past months by the municipality to evacuate the building because of safety concerns."

"We had repeatedly warned the building managers about the lack of safety," Maleki told state TV. "The building lacked fire extinguishers... But the building managers ignored the warnings."

The owner of a nearby grocery store, forced by police to leave the area, told Reuters by telephone: "It was like a horror movie. The building collapsed in front of me."

The semi-official Fars news agency said police had cordoned off the nearby British, German and Turkish embassies. "The flames could be seen kilometers away from the old building," it said.