At least three people died when an explosion ripped through a building in the center of Madrid, the city's Emergency Information Office said in a tweet Wednesday.
A further eight people were injured in the blast on Calle de Toledo, one seriously, it added.
Madrid's Mayor Jose Luis Martinez Almeida told reporters at the scene that initial investigations suggested that the explosion had been caused by a gas leak.
A technician who was working on the boiler is missing, according to the Spanish government's representative for the Madrid region, said Jose Manuel Franco.
The Madrid general vicar, Avelino Sevilla, told Spain's TVE that one person linked to the Catholic Church occupying part of the building was still missing.
The blast caused substantial damage to the six-story building, images and footage posted to social media by the city's emergency services showed.
The top five floors were totally destroyed, with walls blown out, while the bottom two floors were still mostly intact, but charred in places from the flames. Rubble and dust covered the building's immediate vicinity.
Emergency crews could be seen aiding several people on the ground.
Witnesses told Reuters that a plume of smoke could be seen after the blast which affected at least four floors of the building, the Emergency Information Office tweeted.
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Leire Reparaz, who lives in the area, told the Associated Press that she heard a loud explosion shortly before 3 p.m. local time (9 a.m. ET).
"We didn't know where the sound came from,” said Reparaz, 24. “We went up the stairs to the top of our building and we could see the structure of the building and lots of grey smoke.”
Reuters contributed to this report.