Metropolitan Manila police chief Geary Barias said a lawmaker's driver was killed, while ABS-CBN TV network said a number of cars were damaged and at least three lawmakers were among those injured.
The blast happened shortly after 8 p.m. (1200 GMT) as the House ended its session, officials said.
"I heard it and I felt the blast although I was on the other side of the building. The ceiling of the canopy near the south wing entrance came down," Rep. Teodoro Casino told The Associated Press.
Police have cordoned off the area in suburban Quezon City and were looking for clues to the blast.
"We are deeply disturbed and the speaker would like to condemn what happened. We'll make final determination what happened," said House spokesman Noel Albano.
He said Speaker Jose de Venecia was on his way to the site of the blast.
The blast occurred amid heightened political tensions in the country. President Gloria Macapagal Arroyo is facing a third impeachment complaint in as many years amid a barrage of political storms, corruption scandals and a feisty opposition.
The Philippine capital has been jittery since last month, when an explosion damaged a shopping mall in the financial district, killing 11 people and injuring more than 100. A preliminary police report said it was an accident.
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