The blast sent bits of concrete flying across the parking lot.
"It looks like there were a number of people injured," said Joel Villanueva, a congressman.
At least two members of the parliament are thought to be among the injured.
Al Jazeera's Marga Ortigas, reporting from the capital Manila, said the police have set up checkpoints in many surrounding areas following the blast.
Lorenzo Tanada, another congressman, told Al Jazeera that the blast could be heard as far as 10km away.
"It was such a loud explosion," he said.
A blast last month tore through a shopping mall in Manila's financial district, leaving 11 people dead and injuring over 100.
Police initially thought a bomb was to blame, but later said the explosion was an industrial accident.