"A 26-year-old woman was seriously burned" and taken to hospital, the Marseille fire service said. "Three other people were slightly affected by smoke" and also hospitalised after the attack around 9:00 pm (1900 GMT), it added.
The woman's life was in danger after she suffered burns to nearly 60 percent of her body, a spokesperson for the hospital said.
Three adolescents pried open the bus doors and spilled a flammable liquid before striking a match, police said citing witnesses.
Marseille prosecutor Jacques Beaume called the attack "a real ambush".
"The bus was doused with gasoline and then set on fire," he said. "A passenger is between life and death (...) her condition is very serious."
Beaume called for more witnesses to come forward as of the "dozen passengers only three, four or five have been identified. "We need their testimony," he said.
The attack happened in an area on the outskirts of France's second city which firefighters said did "not have a particular record of discourteousness or violence".
It came as France marked the first anniversary of the country's most serious riots in decades which passed without major incidents, with six policemen suffering minor injuries and two buses torched overnight on Friday as well as scattered skirmishes with youths. Four thousand extra police were drafted into Paris suburbs with high immigrant populations and 47 people were arrested, 34 of them in areas around the capital.
On Friday, masked youths in the northeastern Paris suburb of Blanc-Mesnil had attacked buses, forcing the passengers to get off before setting fire to the vehicles.
Police confronted youths in Clichy-sous-Bois, the poor northeastern Paris suburb from where riots spread last year to other areas of the country.
On Saturday, three policeman were injured as dozens of youths attacked security forces with stones and Molotov cocktails at a housing project in the Paris suburb of Grigny, police said.
Grigny's gritty housing project has been the recent target of youth unrest. Youngsters torched a bus last Sunday, causing no injuries, and police and other cars were stoned on Wednesday. On Friday, a total of 277 vehicles were set on fire across the country, according to the Le Monde newspaper, but the authorities refused to confirm the figure.
That figure was three times higher than the number of vehicles torched on a normal day in France.
Police said police and youths had also clashed in the northeastern city of Reims and in Toulouse in the southwest.
Last year's riots were sparked by the deaths in Clichy-sous-Bois of Zyed Benna, 17, and Bouna Traore, 15, both from families of African descent.
They were electrocuted as they hid in an electricity sub-station while fleeing from police.
Night after night for three weeks, mainly north African youths clashed with police, torching more than 10,000 cars and firebombing 300 buildings in around 275 towns, until order was officially restored on November 17.
On Friday, more than 1,000 people made a silent march past the spot where the two teenagers died.
Police and mayors have warned that the conditions that led to the riots remain firmly in place in poor out-of-town neighbourhoods plagued by unemployment of 30 to 40 percent.

Copyright AFP (Agence France-Presse), 2006