Public prosecutors say the driver was a victim "of extreme violence" when he asked to check the ticket of one of suspect and demanded that three others wear facemasks.
Two men have been charged with the attempted murder of a bus driver in Bayonne, who was left brain dead after a dispute with passengers over wearing face masks.
The suspects, aged 22 and 23, are known to police and have been imprisoned on suspicion of the violent assault.
Two other men in their thirties have been charged, the first for "evading arrest and search and failing to assist a person in danger" and the second for "failing to assist a person in danger."
The attack took place early Sunday evening at a bus stop in southeast France.
A 59-year-old bus driver was the victim of an assault "of extreme violence," said the deputy public prosecutor of Bayonne, Marc Mariée.
Authorities say the driver wanted to check the ticket of one of the suspects and demanded that three others wear facemasks.
Wearing a mask on public transport is compulsory in post-lockdown France.
"Insults were hurled and then there was a stampede [and] the driver was pushed out of the bus," the magistrate said at a press conference on Tuesday.
"Two of the individuals give him violent kicks and fists in the upper part of the body and especially towards his head."
The four men left the driver "unconscious on the sidewalk" before fleeing to "take refuge in the apartment of one of them.
The attack aroused strong emotion among the driver's colleagues, who exercised their right to withdraw from work on Monday.
Bus drivers in Bayonne said they would not return to work "before the funeral."
An inter-union has called for France's transport network to stop for a minute of silence at 19.30 on Wednesday.
"The French government stands alongside everyone - employees, elected officials, operators - to secure operating conditions," said the country's new Transport Minister, Jean-Baptiste Djebbari, on Twitter.
The Chronoplus bus network, which serves Bayonne and the neighbouring towns of Biarritz and Anglet, was severely disrupted on Monday following the incident, according to the network's website.
The mayors of Bayonne and nearby Anglet have also assured drivers of their "full solidarity."
"We have witnessed a particularly violent and barbaric act," said Jean-René Etchegaray, Bayonne's mayor.
Etchegaray added his hope "that the perpetrators will be brought to justice and that the punishment will be severe" to send "a significant sign to the entire population."