As most passengers were reclined in their seats asleep, they avoided worse injuries in the accident near Lille in France. REUTERS

The roof of a bus was shorn off leaving many passengers injured when the vehicle tried to pass under a low bridge in northern France.

Six of the injured are in a serious condition, according to the public prosecutor of Lille.

Most injuries were experienced by those in the rear part of the bus as the ploughed debris was thrust to the back.

The bus travelling from Spain was carrying 58 passengers, mostly students who were asleep when the accident happened.

Running between Bilbao and Amsterdam, the bus attempted to drive beneath a bridge prohibiting vehicles exceeding 2.6 metres in height, reports AFP.

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

"All of a sudden, the roof of the tunnel was right there," she said, gesturing with her hand just above her head.

The crash, which occurred at 5.30 am local time, was "extremely violent", the head of the medical team, Francois Ducrocq, told Spanish news group EFE.

However, it could have been much worse if the passengers were not sleeping. As they were reclining, their heads did not extend above the seats, he noted.

The 59-year-old driver of the bus with good credentials said that he had been following instructions provided by his GPS. Tests showed negative for alcohol and drugs.