The pilgrims were at the shrine of Notre-dame-de-la-Salette Enlarge Image

The coach, carrying mostly elderly Roman Catholic pilgrims from Poland, fell into a ravine while descending a steep incline between Grenoble and Gap.

It broke through the safety barrier, fell some 15m (50ft) onto the banks of a stream, and burst into flames. About 20 passengers were also injured.

The coach was returning from the shrine of Notre-dame-de-la-Salette.

The coach, which came from Poland's north-western Szczecin area and was on its way back, was carrying 49 passengers and two drivers.

'Burned to death'

Skid marks on a long stretch of tarmac suggest that the brakes failed.

Pictures of crash scene

Local people brought buckets of water to put out a fire on the bus before emergency services arrived in force.

"When the bus was burning, there were injured people inside," local resident Philippe Baret was quoted as saying by the Associated Press news agency.

"I saw at least six of them who were stuck inside the bus and burned to death before my very eyes," he said.

The injured were taken to hospital by ambulance and helicopter.

Polish President Lech Kaczynski flew to Grenoble to visit the injured in hospital later on Sunday, saying the accident was "one of the most serious disasters in the history of Poland".

French President Nicolas Sarkozy, who accompanied him, said he was "overwhelmed by the scale of the catastrophe".

"During this ordeal, you have the solidarity of the French people," Mr Sarkozy said.

A number of people are reported to be missing. Passengers may have been thrown out of the bus and into the river, firefighters said.

Hugh Schofield in Paris says the busy mountain road is known to be dangerous and there have been several accidents there in the past.

Investigators will want to know if the Polish coach was in a proper state and if it was fitted with the electronic braking system compulsory for excursions into the mountains, our correspondent adds.