An explosion from a gas leak at a clinic in northern Tehran killed at least 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.
The state-run Irna news agency quoted Mr Maleki as saying the dead were 15 women and four men.
He said firefighters had rescued 20 people from the medical centre.
Firefighters were still battling the blaze late into the evening.
Video posted online appeared to show more than one explosion and thick black smoke rising from the flames.
Hamidreza Goudarzi, Tehran’s deputy governor, said a leak from medical gas tanks in the building caused the explosion and fire.
Local media reported the explosion occurred at the Sina Athar Medical Centre in Tajrish.
People in nearby Tajrish Bazaar rushed to the site, impeding a rescue operation, authorities said.
Videos on social media showed people gathered outside the building.
At least 10 ambulances were sent to the site and a nearby hospital went on alert to receive injured people. Police blocked off all roads to the site.
Authorities said firefighters were trying to move people from nearby buildings to safety.
State TV said there could be more explosions because there were a number of oxygen tanks remaining in the medical centre.
Updated: July 1, 2020 11:55 AM