The interior minister, Nicolas Sarkozy, announced on Thursday that he was drafting in extra police to deal with any further unrest after three buses were set alight on Wednesday night.

In one incident, an assailant held a pistol to the driver's head while others forced passengers off.

In Grigny, a suburb south of Paris youths threw stones at police and set on fire a car on Thursday night.

Police say violence has been building ahead of Friday's first anniversary of nationwide riots in which youths from mainly immigrant backgrounds burned cars and wrecked shops.

"I have decided to call up all of the mobile forces that are available to me in order to serve those who take public transport," Sarkozy told reporters after an emergency meeting with transport chiefs at his ministry.

Traffic fear

In January, Sarkozy announced hundreds of police would patrol French trains as part of a new force intended to stamp out violent crime on the network.

Transport bosses had earlier on Thursday threatened to pull traffic off routes in some troubled neighbourhoods in response to arson attacks on their vehicles.

Unrest in the suburbs, where many people face unemployment, poverty and alleged discrimination, could feature prominently in next spring's presidential elections for which Sarkozy is the conservative frontrunner.

Dominique Strauss-Kahn, a possible socialist challenger next April, called for Sarkozy to withdraw police reinforcements from the most volatile suburbs.

"If you want to restore calm in the coming days, if you don't want the anniversary of the events of a year ago to be a drama for France, you must make a signal," he said.

Diversity

Youths burned cars and wrecked
dozens of shops last year

Dominique de Villepin, the prime minister and another Sarkozy rival who many believe could yet challenge the interior minister, promised "exemplary punishment" for the perpetrators.

"We cannot accept the unacceptable ... We refuse to see no-go zones created in our country," he said.

Azouz Begag, the government's equal opportunities minister, called for a celebration of French diversity and played down the anniversary.

"I don't want to talk about an anniversary ... there is nothing to celebrate. Celebrate what, 13,000 cars burned, schools set aflame, violence?"

"The France of diversity is becoming a reality ... this didn't exist before. Even the word 'diversity' wasn't part of the French political debate."