French Police Report Foiling Bomb Plot : Terrorism: Authorities hail arrests as ‘decisive step’ toward ending attacks linked to Algerian rebels.

In their first major breakthrough in combatting the terrorist wave gripping France, police said Thursday they had foiled a bombing planned for this weekend and arrested several people, including an Algerian man overheard giving orders to plant the device.

Jean-Louis Debre, the French Interior minister, hailed the arrests as “a new, decisive step . . . toward neutralizing this group of terrorists.” But, he added, “The threat remains. More attacks can still occur.”

Police raided homes in Paris, Lille and Lyon on Wednesday night and Thursday, seizing guns, grenades and computers. They also found a homemade bomb apparently intended to be detonated, perhaps in a car, at an open-air Sunday market in Lille, in northern France. Debre identified one of the 10 suspects in custody as “a central figure” in a three-city terrorist network.

The announcement was sure to give a boost to French efforts to end the carnage. The French government, which has stationed tens of thousands of heavily armed soldiers around Paris, has been increasingly frustrated by its inability to make important arrests or prevent the attacks that have killed seven and injured more than 170 since July.

The new round of raids and arrests also added credence to the investigators’ theory that the bombs are the work of an international network of Islamic militants trying to force Paris to end economic support for the military-backed regime in Algeria, a former French colony. Some analysts have suggested, though, that the Algerian military could also be staging some of the attacks to discredit its Islamic opponents.

While some anti-terrorism specialists said the police operation this week had broken “the nerve system” of the terrorist network, other analysts warned Thursday that it will take more police work to prevent attacks. Investigators believe that the network includes several loosely linked groups of guerrillas operating in the immigrant suburbs of Paris, Lyon and Lille.

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“Everyone must understand that the danger isn’t over,” French Justice Minister Jacques Toubon told a television interviewer Thursday. “And we must continue to be especially vigilant.” French officials said anti-terrorist measures, including police foot patrols in subway stations, will continue.

Most of those arrested in the sweeps were young, Algerian-born men with rap sheets for petty offenses. But the most important, Boualem Bensaid, alias Mehdi, is a 27-year-old Algerian who arrived in France several months ago. Le Monde, the respected Paris newspaper, said police considered Bensaid an intermediary among terrorist groups with links to Algeria’s Armed Islamic Group, or GIA.

Police said Bensaid also had been in contact with another prime suspect in the bombings, Khaled Kelkal, who was gunned down by police near Lyon in September. Kelkal’s fingerprints were found on an unexploded bomb planted on a high-speed train line between Lyon and Paris in August.

Bensaid, who lives in an apartment in a chic Paris neighborhood, was arrested Wednesday night after French authorities allegedly heard him, in a tapped telephone conversation, order his compatriots in Lille to plant a bomb Sunday in that city’s crowded Wazemme market. Police had been following Bensaid for several days.

Police then raided a three-room suburban Lille apartment that had been rented last month by a young French student and was being used as a meeting place for Islamic militants. Among the items seized there was a 29-pound gas canister filled with explosive substances and pieces of metal, and a timing device made from an alarm clock. The bomb was nearly identical to those used in several of the eight previous attacks.

The bombing campaign in France began over the summer, shortly after the assassination of a moderate Muslim cleric in Paris. Since then, bombs have exploded in two subway trains and on city streets, as well as in front of a Jewish school.

The GIA, Algeria’s most violent Muslim guerrilla group, has claimed responsibility for most, though not all, of those attacks. Among other things, the GIA has demanded that France end its support for the Algerian regime and that French President Jacques Chirac convert to Islam.

The frequency of the attacks in France, as well as in Algeria, has increased in the run-up to the Nov. 16 Algerian presidential elections. Liamine Zeroual, a former general who was appointed president, is expected to win the elections, which most Islamic groups are boycotting.