One building collapsed in a central Madrid explosion, a Reuters reporter saw, with smoke coming out of the building and rescue workers evacuating elderly people from a nearby nursing home.
A reporter for TVE public broadcaster said several had been injured, without giving details. A witness told Telemadrid broadcaster that there was at least one person trapped inside.
Emergency services could not immediately confirm if there had been injuries.
A police spokeswoman said the area was being evacuated but could not confirm the source of the explosion.
La Sexta TV quoted government sources saying it may have been a gas leak. Officials could not immediately confirm.
The explosion happened in Toledo street, in the city center.