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.
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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.
Analysis Continue reading the main story Marco was flamboyant on and off the track. When someone dies, everyone always says they loved life. But he had a very vibrant personality. He already had a huge fanbase around the world, partly down to aggressive riding - but also because he was just a cool guy. He didn't take himself too seriously and would have been a big star for next year. Matt Roberts BBC MotoGP presenter Read Steve Parrish's tribute
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."
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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.
Simoncelli en el Gran Premio de Australia, realizado la semana pasada, y donde terminó segundo.

La que iba a ser la penúltima carrera del Campeonato del Mundo de Motociclismo de 2011 en MotoGP, llevada a cabo hoy en el Circuito Internacional de Sepang, al sur de la península de Malaca, en Malasia, terminó en tragedia con la muerte del italiano Marco Simoncelli, piloto de Honda.

Simoncelli, que contaba con 24 años, falleció a las 16:56 (hora local), tras caer en la segunda vuelta, ser atropellado por el estadounidense Colin Edwards y el también italiano Valentino Rossi, y permanecer inerte sobre el asfalto. Dada la fuerza del impacto, el casco del piloto logró salir despedido y saltar varios metros lejos del lugar del accidente. Por razones de seguridad, se ondeó la bandera roja inmediatamente.

De acuerdo a las declaraciones de Michele Macchiagodena, quien se encargó de encabezar al equipo médico del circuto, “cuando nuestro equipo médico ha llegado a él estaba inconsciente. En la ambulancia, debido a que sufría un paro cardiaco, se ha comenzado la reanimación cardiopulmonar”, agregando que los intentos para salvarlo se prolongaron durante 45 minutos, tras lo cual se le declaró muerto.

Entretanto, Edwards, que también cayó, se dislocó un hombro, mientras que Rossi y los demás competidores lograron permanecer sobre sus motos, saliendo ilesos. El español Maverick Viñales y el suizo Thomas Lüthi ocuparon la primera posición en las categorías 125cc y Moto2.

Esta muerte es la primera en MotoGP desde las de los japoneses Daijiro Kato y Shoya Tomizawa, ocurridas en Japón y San Marino en las temporadas de 2003 y 2010, respectivamente.

La última etapa se realizará entre el 4 y 6 de noviembre próximos en Valencia.