The Malaysian MotoGP was cancelled four minutes after the race began on Sunday when Honda rider Marco Simoncelli fell and was hit by Colin Edwards and Valentino Rossi.
The 24-year-old Simoncelli had his helmet knocked off in the accident, which occurred when the Italian lowsided from his bike at Turn 11.
His bike veered across the track and straight into the path of Edwards and Rossi. The race was immediately red-flagged.
Simoncelli, who was motionless on the track after the impact, had been fourth at the end of lap one.
Edwards also fell but escaped serious injury, while Rossi, on a Ducati, was able to return to the pits.
Simoncelli's condition was not immediately known.
Newly-crowned champion Casey Stoner admitted watching the accident made him worried for Simoncelli.
"As soon as I saw the footage it just makes you sick inside," the Australian told BBC2.
"Whenever the helmet comes off that's not a good sign."
Tony Elias, Moto2 world champion, was praying the injuries to Simoncelli were not serious.
"We just hope Marco is ok - that's my wish," he said.