An explosion from a gas leak in a medical clinic in northern Tehran killed 19 people, Iranian state TV reported 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 Maleki as saying the dead included 15 women and four men. 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.
Hamidreza Goudarzi, deputy Tehran 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 neighbourhood.
Associated Press writer Nasser Karimi contributed to this report.