Six passengers have been seriously injured in a spectacular accident in northern France when the top of a bus was shorn off while trying to pass under a low bridge.

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 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 about 5.30am local time on Sunday.

"Most people were sleeping... [and] nobody understood what was happening," said a passenger from Spain's Basque region identifying herself as Carlota.

"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.

Mr Fevre said the 59-year-old driver of the bus had a good record and assured authorities he had followed instructions provided by his GPS.

The driver was held for questioning on Sunday and authorities said testing for alcohol and drugs had come back negative.

Passengers who did not require hospitalisation were being put up in a local sports centre, waiting for repatriation to Spain.

Francois Ducrocq, a psychiatrist working with an emergency aid team assisting the victims said: "They're resting, we know stress is exhausting. They are together [and] have called their families... their return [home] is being organised".

AFP