Italian rider Marco Simoncelli has died after a horrific crash at the Malaysian MotoGP in Sepang.
The race was stopped on lap two when Simoncelli's bike veered across the track at turn 11 into the path of Colin Edwards and Valentino Rossi.
The 24-year-old had his helmet knocked off and was hit by both other riders.
American rider Edwards also fell and dislocated his shoulder - while Simoncelli's fellow Italian Rossi was able to return to the pits.
Simoncelli lay motionless on the track after the impact, while the race was immediately red-flagged.
Medical director Michele Macchiagodena said Simoncelli suffered a "very serious trauma to the head, to the neck and the chest".
At first officials were looking to restart the race before the extent of Simoncelli's injuries became clear.
The race was cancelled amid confusion in the grandstand, with fans throwing bottles to show their initial displeasure.
The death is the first fatality in MotoGP since Japan's Daijiro Katoh died from injuries sustained at the 2003 Japanese GP while, last year, Japan's Shoya Tomizawa died in a similar crash to Simoncelli in a Moto2 race in San Marino.
Simoncelli's death comes a week after British IndyCar driver Dan Wheldon was killed in Las Vegas following a 15-car crash.
BBC MotoGP presenter Matt Roberts said: "Marco lost his helmet in the crash. The officials said that, when the track medics got to him, he was in cardiac arrest. They tried to resuscitate him in the ambulance and the medical centre.
"Both riders collided with him and the impact corresponded to him losing his helmet. The saddest thing is that Valentino [Rossi] and Marco were very close friends.
"Colin [Edwards] has a dislocated shoulder and is in a lot of pain. He and Valentino are absolutely devastated."
A MotoGP statement read: "On Sunday, 23 October, during the MotoGP race at the Sepang International Circuit, San Carlo Honda Gresini's Italian rider Marco Simoncelli suffered a serious accident wherein he sustained critical injuries.
"The race was stopped immediately with the red flag and Simoncelli was transported by ambulance to the circuit medical centre where the medical staff worked to resuscitate him.
"Despite their efforts, Marco sadly succumbed to his injuries at 4.56pm local time [0956 BST].
"Everybody involved in MotoGP extends its deepest condolences to Marco's family, friends and team at this tragic loss."
And Sepang circuit chairman Mokhzani Mahathir added: "This is a one-of-a-kind freak incident where the helmet came off and I am sure FIM [International Federation of Motorcycling] and MotoGP will be looking into this."
Honda rider Simoncelli entered MotoGP for the 2010 season and won his first pole position at the Catalunya race in June this year.
Simoncelli's first podium finish came in the Czech Republic in August when he finished third but he bettered that with a second-place finish at the Australian GP.
World champion Casey Stoner said: "As soon as I saw the footage it just makes you sick inside. Whenever the helmet comes off that's not a good sign."
British MotoGP rider Cal Crutchlow tweeted: "RIP Marco Simoncelli! A great rider and all round nice guy. My thoughts are with all his family & friends. I will never forget today."
Australian Formula 1 driver Mark Webber tweeted: "RIP Marco. A special talent that will be missed. Thinking of your loved ones and all the MotoGP paddock."
Major Italian sports events on Sunday observed a minute's silence in memory of Simoncelli, who was a big fan of football club AC Milan.
In their Serie A fixture against Lecce, Milan wore black armbands and came back from three goals behind to win 4-3.