French riot police reinforcements have been sent to Marseille after overnight violence left a woman severely burnt and fighting for her life.
The woman was injured when youths set a bus alight during the night in the southern city.
The attack sparked fears of more violence in French cities, coming as France marks the anniversary of riots that scarred the nation's poor, largely immigrant suburbs.
President Jacques Chirac condemned the latest unrest.
"The president told the victim's family about his horror concerning this shameful act and assured them everything would be done to find the criminals and punish them with the utmost severity," Chirac's office said in a statement.
Local officials said some extra 160 officers were being sent to Marseille, a city which remained largely untouched by last year's riots. They said police would enforce a 'zero tolerance' policy in troublesome neighbourhoods.
Police said four youths forced their way onto the bus and torched it before a French woman of Senegalese origin could escape. Officials said the 26-year old was "between life and death", with burns covering 60 percent of her body.
"What is frightening ... is that (these people) are not just attacking police - representatives of the state - but are attacking the population," Jean-Claude Delage from police union Alliance told LCI TV. "Apparently, they did not even ask people to get off the bus (before torching it)."
Vandals have torched at least half a dozen buses in suburbs surrounding Paris this past week in increasing violence ahead of the anniversary, but no one was hurt in those attacks. Police have warned the violence could once again spiral out of control.
Overnight, youths set another bus ablaze in the Paris suburb of Trappes after ordering passengers off. Vandals lobbed stones at police in the suburbs of Clichy-Sous-Bois and Montfermeil. Police said they arrested 46 people. Two officers were injured.
The latest violence is adding pressure on the conservative government ahead of 2007 presidential and parliamentary polls.
Local politicians and many youths in the high-rise suburbs say the conservative government has failed to tackle the root causes of the riots, such as unemployment and discrimination.