PARIS — Five teenagers, aged between 15 and 17, were arrested yesterday in the southern French city of Marseille on suspicion of carrying out an arson attack on a bus that left a young Senegalese woman with horrific burns to most of her body.

The age of the suspects, as well as the casual savagery of the attack, has shocked a French public and political class that had almost grown numb to reports of violence in the run-down housing estates of the country’s largest cities.

Those arrested were identified by witnesses to Saturday’s attack and by neighbors, said Jacques Beame the Marseille state prosecutor.

Marseille had prided itself on avoiding the urban violence between youths and police that blight the suburbs of cities further north, such as Paris and Lyon.

When police officers raided the homes of the youths before dawn, residents did nothing to prevent the arrests. Those arrested could face up to 30 years in prison if convicted of the attack.

The arsonists attacked the bus partly in revenge for a perceived slight, investigators said. The female driver had refused to allow them to board between stops. When she returned to their neighborhood, they forced their way on, threw gasoline inside the vehicle, and lit it.

The driver led most passengers to safety, but gasoline had soaked the clothes of Mama Galledou, who caught fire.

Ms. Galledou had recently obtained a degree in nutrition at a Marseille university, where around 250 students and teachers held a rally in her support.

Ms. Galledou was placed on a respirator, in a medically induced coma to avoid exposing her to intense pain. If she survived, she would be badly handicapped and disfigured, doctors said.

The attack was the most violent of a string of attacks by youths on public buses. With more than 100 cars set alight on an average night in France, the bus attacks represent an escalation of violence.