“The blast hit the lower floors of the downtown tower block, throwing debris into the streets and sending workers running outside.
Interior Minister Miguel Angel Osorio Chong said the blast killed at least 14 people and injured 100. It was not yet clear what caused the explosion, and the death toll could still rise, he added.
Media reports said machinery inside the building had apparently exploded. But an ambulance service official at the scene, who asked not to be named, said it was caused by a gas leak.
Some people at the scene said the blast came from a neighboring building.
Pemex said initially its headquarters had been evacuated because of a problem with its electricity supply. It then said there had been an explosion, but did not say what caused it.