An explosion at the main headquarters of Mexico's state-owned oil company in the capital killed 25 people and injured 101 on Thursday as it heavily damaged three floors of the building, sending hundreds into the streets and a large plume of smoke over the skyline.

Rescuers continued to search the rubble for victims trapped in the debris late Thursday with the aid of rescue dogs, trucks with mounted lights and a crane.

Interior Minister Miguel Osorio Chong said it was uncertain if there were any people still trapped but that crews would keep searching. Many of the office workers were outside having lunch when the explosion occurred about 3:45 p.m. local time in a basement parking garage next to the iconic, 51-storey tower of PetroleosMexicanos, or Pemex, one of the tallest buildings in Mexico City.

Osorio Chong said the explosion hit the basement and first two floors, which rescuers said had collapsed.

"It was an explosion, a shock, the lights went out and suddenly there was a lot of debris," employee Cristian Obele told Milenio television, adding that he had been injured in the leg. "Co-workers helped us get out of the building."

President Enrique Pena Nieto said authorities have not yet found what caused the blast in the 14-storey building in a busy commercial and residential area. Pemex first said it had evacuated the building because of a problem with the electrical system. The company later tweeted that the Attorney General's Office was investigating the explosion and any reports of a cause were speculation.

Ana Vargas Palacio was distraught as she searched for her missing husband, Daniel Garcia Garcia, 36, who works in the building. She last heard from him at 1 p.m. CT.

"I called his phone many times, but a young man answered and told me he found the phone in the debris," Vargas. The two have an 11-year-old daughter. His mother, Gloria Garcia Castaneda, collapsed on a friend's arm, crying "My son. My son."

An injured person is transported on a stretcher at the headquarters of state oil giant Pemex in Mexico City on Thursday. (Alejandro Dias/Reuters)

The tower, where several thousand people work, was evacuated following the blast but not damaged, according to Gabriela Espinoza, 50, a Pemex secretary for 29 years who was on the second floor when the explosion next door occurred.

"There was a very loud roar. It was very ugly," she said.

Espinoza's co-worker, Tomas Rivera, 32, works on the ground floor and was knocked to ground, fracturing his wrist and jaw.

Hundreds of firefighters, military in camouflage and Red Cross workers hauled large chunks of concrete and looked for victims late into the night, with at least four bodies pulled out of the rubble, according to an Associated Press reporter at the scene.

The exploded building was intact on the outside but filled inside with debris.

Television images showed people being evacuated in office chairs, and on gurneys. Most of them had injuries likely caused by falling debris.

'Smoke and glass'

"We were talking and all of sudden we heard an explosion with white smoke and glass falling from the windows," said Maria Concepcion Andrade, 42, who lives on the block of Pemex building. "People started running from the building covered in dust. A lot of pieces were flying."

Police landed four rescue helicopters to remove the dead or injured. About a dozen tow trucks were furiously moving cars to make more landing room for the helicopters.

"I profoundly lament the death of our fellow workers at Pemex. My condolences to their families," Pena Nieto said via Twitter. He later toured the scene.

Streets surrounding the building were closed as evacuees wandered around, and rescue crews loaded the injured into ambulances. The injured were taken to Pemex's hospital in Azcapotzalco and the Red Cross hospital in Polanco, where relatives huddled in the waiting room for news of their loved ones. Some walked out of meetings with the hospital social worker joyous, while others came out crying

Pemex, created as a state-owned company in 1938, has nearly 150,000 employees and in 2011 produced about 2.5 million barrels of crude oil a day, according to its website, with $111 billion in sales.

Shortly before the explosion, Operations Director Carlos Murrieta reported via Twitter that the company had reduced its accident rate in recent years. Most Pemex accidents have occurred at pipeline and refinery installations.

A fire at a pipeline metering center in northeast Mexico near the Texas border killed 30 workers in September, the largest-single toll in at least a decade for the company.