PARIS — The French authorities began to use new, sweeping counterterrorism powers on Monday to halt the stream of young people going to Syria, confiscating the passports of six people suspected of trying to go there to engage in terrorist activities.
The interior minister, Bernard Cazeneuve, announced that the ministry had taken six people’s passports under the new law and that the ministry was preparing travel bans for 40 more people.
“If French citizens leave to commit atrocities in Iraq and in Syria, when they return, they represent an even bigger danger for the national territory and they are likely to commit terrorist acts on a major scale,” Mr. Cazeneuve said.
The law, approved in November, includes a provision that allows the French authorities to effectively impose a travel ban on anyone they suspect of leaving the country for the purpose of engaging in terrorist activities. The ban lasts six months, at which point the government can renew it every six months for up to two years. The ban can be appealed.
The provision was written and debated in 2014 in response to concern about the hundreds of French citizens who either had departed for Syria or Iraq, or were trying to. In using it, France is on a path similar to at least two other European countries, Britain and Denmark, which have recently approved measures allowing them to take away citizens’ passports.
But more would-be jihadists have traveled from France to Syria than from any other European country. Manuel Valls, the French prime minister, said that the authorities estimated that about 1,400 French citizens were involved in networks funneling potential fighters to Syria or Iraq.
Under the similar British measure, the police or border officials can seize for up to 30 days the passports of people suspected of trying to join a terrorist group regardless of whether they are trying to leave the country.
Another provision of the British law allows the cancellation of travel documents while people are abroad if they are suspected of participating in terrorist activities. Such people could eventually return, but only after proving that they are no longer dangerous.
The new law in France has not been much questioned in the wake of the January attacks on the satirical newspaper Charlie Hebdo and on a kosher supermarket, which were frightening to many in France who had not expected such terrorism to happen here.
However, lawyers who specialize in civil rights laws said they were worried about how the law would be used and the potential for the government to apply it too broadly and make mistakes.
“This is clearly a violation of a fundamental right which is the liberty of free movement,” said Agnès Tricoire, a lawyer who specializes in intellectual property and freedom of expression and represents the French League of Human Rights.
But, she added that since these were the first cases of passport confiscation under the new law, it was too soon to make predictions about how the government would use the law.
Ms. Tricoire expressed concern about the unintended consequences of halting the departure of highly motivated would-be fighters, echoing a point raised by civil rights lawyers in Britain during the debate over its new counterterrorism rules.
“One of the issues is what are they going to do if they can’t leave,” she said.