The 33-year-old Australian, champion for the past five years, was catapulted off his Honda on a fast, left-handed turn when the bike apparently clipped a kerb still slippery after morning rain.
Observers estimated he was doing about 110mph when the front wheel wobbled and threw him off. "It just looked like he was pushing too hard and went over the limit," said fellow rider Carlos Checa.
A Spanish source quoted medical officials as saying Doohan had broken the tibia and fibia of his left leg. After first being taken to the track medical centre, he was taken to hospital.
While Doohan was sure to miss tomorrow's race, there was no immediate comment from his team or doctors on how many weeks he might be out of action.
Doohan lay prostrate in the gravel trap after the crash, fortunate to escape being hit by his machine, which hurtled through the air into the fencing along the track. A security guard was also reportedly hit by the flying debris and taken to hospital.
The first practice session was immediately halted after Doohan's accident, but restarted later with his team-mate, the Spaniard Alex Criville, taking provisional pole.
Doohan is second in the world championship with 33 points, behind Kenny Roberts Jnr on a maximum 50 after the 25-year-old American won both races this season for Suzuki.
Doohan, whose girlfriend Selina is expecting their first child, is the second most successful 500cc racer of all time, with five world titles and 54 race wins out of 136 starts.
He said last year that he still loved racing but this season, in which he faces his toughest challenge yet, could be his last: "I think new century, new job." he said.Reuse content