French President Jacques Chirac warned of a "dangerous situation" and called for calm after six nights of riots in suburbs in the north-east of Paris.
Violence in Aulnay-sous-Bois caught police unaware
At least 15 cars were torched overnight in Aulnay-sous-Bois. Police fired rubber bullets and arrested 34 people.
Unrest flared after two North African teenagers were electrocuted. Locals say they were fleeing police, which the authorities deny.
"The law must be applied in a spirit of dialogue and respect," Mr Chirac said.
"A lack of dialogue and an escalation of disrespectful behaviour will lead to a dangerous situation," he told a cabinet meeting, according to a spokesman.
French Prime Minister Dominique de Villepin and Interior Minister Nicolas Sarkozy have delayed trips abroad to try to calm the situation.
Both met relatives of victims, police officials and community leaders on Tuesday in an effort to calm tensions. But fresh violence broke out even as the pair opened talks, and spread to new parts of Paris.
Police said as many as 69 cars were set on fire on Tuesday night in nine towns in the Seine-Saint-Denis region, home to many impoverished communities.
The original flashpoint of Clichy-sous-Bois, where police were out in force, was calmer, but trouble flared in nearby areas.
Correspondents say anger grew after a tear gas canister was hurled into a mosque in Clichy-sous-Bois on Sunday night. Emotions have also been fuelled by mass arrests.
War of words
Mr Sarkozy, criticised for his description of the rioters as a "rabble", has repeated his condemnation.
"I speak with real words," Mr Sarkozy, who has cancelled a visit to Afghanistan and Pakistan next week, told Le Parisien newspaper.
"When you fire real bullets at police, you're not a 'youth,' you're a thug."
Mr de Villepin has delayed a trip to Canada to try to ease tensions.
He and Mr Sarkozy have been accused of playing politics with the situation in an effort to win favour ahead of a presidential campaign in 2007.
Unrest flared in Clichy after two teenage boys were electrocuted on Thursday at an electricity sub-station.
Local people insist they were fleeing from police and scrambled in to hide. Police say they were not chasing the boys.
An official investigation is under way.
Clichy saw five successive nights of confrontation between police and young people from the mainly north African Muslim communities in the north-eastern suburb.
Unemployment and social problems are rife in many of France's poorer suburban areas.
Police have reported sporadic incidents involving mobile groups of youths in the Val-d'Oise, Seine-et-Marne, Hauts-de-Seine and Yvelines regions of Paris.