Killed in the blast was Wahab Akbar, a congressman representing Basilan island, where troops are hunting down remnants of the Al-Qaeda-linked Abu Sayyaf extremist group. Akbar had received death threats in the past.

Troops went on heightened alert and security forces set up checkpoints around Manila as President Gloria Macapagal-Arroyo quickly ordered a national police probe into the blast at the sprawling complex north of the capital in suburban Quezon City.

The explosion - which occurred just after 8pm, soon after most congressmen had left the premises - destroyed part of the building's south lobby, House Speaker Jose de Venecia told local radio.

"There is a bomb that exploded in the south wing of the Batasan complex," De Venecia said, adding that he had left the building just 10 minutes before the explosion, which sent concrete flying across the parking lot.

"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," Representative Teodoro Casino said.

Also killed was a driver of one of three injured lawmakers. His body was inside a parked vehicle hit by debris.

Arroyo immediately ordered national police chief Avelino Razon to "personally supervise the investigation, determine the cause of the explosion which rocked Batasan," her spokesman Ignacio Bunye told reporters.

"If this is terrorist action or work of an anarchist, I'm sure it was deliberately done to cow us," De Venecia told reporters at the site.

Manila police chief Geary Barias deployed a team to cordon off the building.

"We had not received any intelligence report," of a possible attack, he said.

"We would like to investigate further. There were two vehicles hit by the blast, and we are trying to recover them. We are investigating right now.

"We are trying to determine where the blast came from," he added.

De Venecia said he had ordered a clean sweep of the complex to ensure "there are no other bombs are left behind."

"It was a very huge explosion," said congressman Joel Villanueva, adding that fewer than 50 of the House's 275 members were inside when the bomb exploded. "We are stunned."

Congressman Roilo Golez, a former national security chief, condemned the violence, but refused to speculate as to who might be behind it.

Last month, a blast tore through a shopping mall in Manila's financial district, leaving 11 people dead, more than 100 injured and sparking fears in the capital of further attacks.

Police initially thought a bomb was to blame, but later said the explosion was an industrial accident.

AGENCE FRANCE-PRESSE, ASSOCIATED PRESS