Six suspects were detained on Monday but the main target of the operation, an Indian national who recently arrived from Algeria, was taken on Tuesday, according to officials close to the inquiry.
Interior Minister Claude Gueant had said Monday that France had no concrete evidence of a specific attack being planned, but security forces were in a heightened state of vigilance over the threat.
The first arrests were made on Monday in Paris and two largely-immigrant suburbs: Stains, where searches continued, and Garges-les-Gonesse, officials close to the inquiry told AFP, speaking on condition of anonymity.
The Indian who was arrested on Tuesday was "the main target" and had "links with Pakistan", an official said, also on condition he not be identified.
All the raids were carried out by the DCRI police intelligence service as part of a probe into a network alleged to have recruited militants in France and sent them to Afghanistan and Pakistan.
The suspects are being held under anti-terrorism statutes at the DCRI's headquarters in the Paris suburb of Levallois, where they can be held for questioning for up to four days without magistrates being consulted.
They were identified partly thanks to intercepted Internet communications between suspected members of the militant recruitment network, police said.
French officials told AFP recently that investigators suspect some of these fighters may have recently returned from the battlefield to France.
"On the day after the death of Osama bin Laden I ordered reinforced vigilance," Gueant told reporters on Monday at a joint news conference in Paris with US Attorney General Eric Holder.
Paris residents have noticed a larger than normal security presence around tourist attractions and transport hubs in recent days, but Gueant said he had not ordered an increase in the terror alert level.
France's security status has been at the second highest level, "Red" or "probable threat", since 2005, when suicide bombers struck London and triggered fears of more attacks on neighbouring European capitals.
In 2008, officials reinforced security measures but stopped short of moving to the highest threat level, "scarlet" or "specific threat", which would trigger draconian security measures and the closure of potential targets.
French forces are fighting in Afghanistan as part of the NATO-led coalition battling Taliban guerrillas and Bin Laden's al Qaeda network has repeatedly threatened to carry out attacks against French citizens and interests.
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