An explosion from a gas leak in a medical clinic in northern Tehran killed 19 people, Iranian state TV reported on Tuesday.
Authorities initially said 13 people were dead, but Jalal Maleki, spokesman for the Tehran Fire Department, later told state TV that the toll had risen to 19.
State-run IRNA news agency also quoted Mr Maleki as saying the dead included 15 women and four men. Mr Maleki added that firefighters had rescued 20 people.
Video posted online appeared to show more than one explosion and thick black smoke rising from the flames and another showed firefighters using a ladder to reach the roof of the building of the Sina At’har health centre.
Hamidreza Goudarzi, Tehran’s deputy governor, told state TV that a leak from medical gas tanks in the building was the cause of the explosion and fire.
People in nearby Tajrish Bazaar rushed to the scene, impeding a rescue operation, authorities said.
Videos on social media showed people gathered outside of the building.
State TV said there could be more explosions because there were a number of oxygen tanks remaining in the medical centre.
Witness Marjan Haghighi told The Associated Press that police blocked roads to the neighborhood.
The clinic, which had 25 employees inside at the time of the blast, primarily carries out light surgeries and medical imaging, Ms Harirchi said.
Last week, an explosion occurred close to a sensitive military site near Tehran which the defence ministry said was caused by a leaking tank at a gas storage facility. No deaths or injuries were reported.