"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.