Frederic Fevre, the public prosecutor in the northern city of Lille, said six people were in grave condition after the bus from Spain carrying 58 passengers attempted to drive under a bridge prohibited to vehicles exceeding 2.6 metres (8.5 feet) in height.
Twenty-eight other passengers -- mostly Spanish students aged 18 to 29 -- were slightly injured aboard the bus servicing the Bilbao-Amsterdam route when the accident occurred outside Lille at around 5:30 am (0330 GMT) local time.
"All of a sudden, the roof of the tunnel was right there," she said gesturing with her hand just above her head.
According to witnesses, the most severely injured passengers were located at the rear of the bus, towards which the debris of the roof was thrust as the vehicle ploughed into the bridge.
Also Read
Fevre said the 59-year old driver of the bus when the accident occurred had a good record, and assured authorities he had followed instructions provided by his GPS.
Passengers who did not require hospitalisation were being put up in a local sports centre, waiting for repatriation to Spain.
"They're resting, we know stress is exhausting. They are together (and) have called their families... Their return (home) is being organised," said Francois Ducrocq, a psychiatrist working with an emergency aid team assisting the victims.