The 54-floor Pemex building is 214 metres (702 feet) tallAt least 25 people have died and another 100 were injured in Mexico City Thursday after an explosion at the headquarters of Mexico's state oil company, Pemex.
Emergency services, assisted by dogs, are searching for people trapped in the rubble. The cause of the blast is under investigation, Pemex says.
The 54-floor Pemex Executive Tower houses hundreds of employees.
Last September, 30 people died in an explosion at a Pemex gas plant in northern Mexico.
At least 25 people were killed at the scene while another person died later in hospital, Mexican Interior Minister Miguel Angel Osorio Chong told reporters.
A further 100 people were injured in the explosion. It is not clear how many individuals remain trapped beneath the rubble. Some reports have placed the figure at more than 30.
Television pictures showed debris from the blast spread out on to the street in front of the building, and Red Cross ambulances on the scene attending to the injured, our correspondent in Mexico City reports.
Police have cordoned off the streets around the building, which is located in a busy commercial area of Mexico City.
At this stage there is no official explanation for the explosion, our correspondent says.
Pemex said on Thursday evening it was investigating the possible causes.
Images of the blast posted on Twitter revealed large clouds of smoke billowing from the building. TV footage showed people being transported from the scene by helicopters.
President Enrique Pena Nieto and Mexico City Mayor Miguel Angel Mancera headed to the scene of the blast.
"I am deeply sorry for the deaths of our fellow workers at Pemex. My condolences to their relatives," Mr Nieto said on Twitter.
The president said he has ordered an investigation into the causes of the blast.
Earlier on Thursday, Pemex had reported problems with the electricity in the building in a message on Twitter.
Pemex has experienced a number of fatal accidents in recent years.
Last September's deadly blast at a gas plant near the northern town of Reynosa is thought to have been caused by a build-up of gas.