French police have detained 19 people in a crackdown on suspected Islamist extremists in cities around the country.

President Nicolas Sarkozy promised more raids to come.

Tensions are high following the attacks by Islamist Mohamed Merah in southern France that left seven people dead and two wounded and ended up with police killing the gunman last week after a 32-hour stand-off.

Sarkozy gave no details about the reasons for the arrests or what the detainees were suspected of.

"It's in connection with a form of Islamist radicalism," he said.

"There will be other operations that will continue and that will allow us to expel from our national territory a certain number of people who have no reason to be here."

Sarkozy said he did not know whether the 19 detainees were part of any network.

A police investigator said the anti-terrorist section of the Criminal Brigade detained five men before dawn in Paris who had suspected links to an Islamist movement. Weapons were also seized.

The other arrests took place in Toulouse, Marseille, Nantes and Lyon. He said these raids were not linked to the inquiry into the killings in Toulouse and Montauban.

In Nantes, the head of Forsane Alizza, a radical Muslim group that formed two years ago, was among the detained. In October 2011, a preliminary inquiry was opened into the Forsane Alizza organisation, and the French Interior Ministry broke up the group in February.

Merah, a 23-year-old Frenchman, claimed responsibility for the shootings that left seven dead and said he had links to al Qaida.

He was buried near Toulouse yesterday.

Three Jewish schoolchildren, three paratroopers and a rabbi were killed in the worst terrorist attacks in France since the 1990s.

French Muslims have worried about a backlash after Merah's attacks, and French leaders have urged the public not to equate Islam with terrorism.

But concerns about radical Islam are high, and the government banned a string of international Muslim clerics from entering France for a conference of a fundamentalist Islamic group.

Sarkozy is locked into a tough battle for re-election ahead of the first round April 22 presidential vote. For years he has made law and order one of his signature themes.

"It's our duty to guarantee the security of the French people. We have no choice. It's absolutely indispensable," he said.