Interior minister Miguel Angel Osorio Chong said 17 women and eight men died in the explosion, that also left 101 injured.
Many casualties remained trapped in the rubble today, as soldiers with rescue dogs and a crane were used in the rescue effort.
The blast wrecked the basement and two floors of the Petroleos Mexicanos, or Pemex, building in Mexico City and sent hundreds fleeing into the street.
The Interior Ministry said it was unsure how many were trapped because staff were outside having lunch at the time of the blast.
"It was an explosion, a shock, the lights went out and suddenly there was a lot of debris," said employee Cristian Obele, who was injured. "Co-workers helped us get out of the building."
President Enrique Pena Nieto said authorities had not yet found what caused the explosion in the base-ment parking garage of the 14-storey building in a commercial and residential area.
Pemex first said it had evacuated the building due to an electrical fault but later tweeted that the Attorney General's Office was investigating and reports of a cause were speculation.
Ana Vargas Palacio was searching for her husband, Daniel Garcia Garcia, 36. She said: "I called his phone many times, but a man answ-ered and said he'd found the phone in the debris."
The couple have an 11-year-old daughter.