No one had claimed responsibility by early this morning, but the target, the timing and the type of bomb echoed attacks last year by Islamic fundamentalists fighting against Algeria's French-backed military rulers.
The French Prime Minister, Alain Juppe, who arrived at the scene at Port Royal station within half an hour of the explosion, confirmed that it was caused by a bomb and immediately invoked the security alert, Vigipirate. The plan, applied across France during last year's wave of attacks from July to October, involves intensive patrols by police, paramilitary gendarmes and soldiers at public-transport stations, at public buildings and in the streets.
Border controls, virtually eliminated under a European Union open-border agreement, were also reinstituted last night.
"From [Wednesday] morning, the Defence Ministry will reinforce surveillance at Paris stations, transport networks and airports," an aide to the Interior Minister, Jean-Louis Debre, said. "A first contingent of 500 men will back the action of police and gendarmerie officers." He added that troops and police would also boost security in cities across the country.
The Interior Ministry confirmed that the explosion had been caused by a 13kg gas canister filled with an unidentified explosive. Investigators found fragments of the canister on the spot, and the body of one of the victims had been mutilated by metal nails.
Yves Bonnet, a former head of France's DST counter-espionage service, said: "Only the Algerian Islamic Armed Group [GIA] threatens France with this kind of attack."
The blast at the junction of Boulevard Montparnasse and Boulevard Saint- Michel occurred just two stops away from the Saint-Michel station, where eight people were killed and 86 wounded in an attack blamed on Algerian extremists in July 1995.
President Jacques Chirac, whose scheduled meeting with Chancellor Kohl of Germany was interrupted by news of the blast, came out into the precincts of the Elysee shortly before 8 o'clock to issue a statement deploring the attack as "unacceptable and barbaric" and saying that he, the government, and all France would unite their forces to fight terrorism.
The only other likely suspect for the Port Royal explosion, apart from Muslim fundamentalists, would be the Corsican nationalists, who have extended their bombing campaign to mainland France over the past two months.