Celebrated Italian rider Marco Simoncelli died on Sunday after a horror crash that saw the Malaysian MotoGP at Sepang cancelled, in the latest tragedy to hit motor sports.

The smash occurred just minutes after the race began, when the mop-haired 24-year-old Honda rider lost control and veered across the track into the path of riders Colin Edwards and Simoncelli's compatriot Valentino Rossi.

Simoncelli, who ironically enjoyed his finest hour at Sepang when he clinched the 2008 season's 250cc title here, had his helmet knocked off in the collision, which left him face down and motionless on the asphalt.

He suffered "very serious trauma to the head, neck and the chest" and later went into cardiac arrest, MotoGP medical director Michele Macchiagodena told reporters.

"Despite their (doctors') efforts, Marco sadly succumbed to his injuries at 4:56pm local time (0856 GMT)," MotoGP said in a sombre statement.

"Everybody involved in MotoGP extends its deepest condolences to Marco’s family, friends and team at this tragic loss."

A distraught world champion Casey Stoner said he was "shocked and upset".

The Australian added: "When something like this happens you remember how precious life is. I feel sick right through, I can only say my thoughts are for his family."

Another rider, Spain's Dani Pedrosa, echoed Stoner's feeling of disbelief.

"After a tragedy like that there's not much you can say. I saw Marco's father and the only thing we could do was to hold each other hard in our arms.

"We often forget this is a dangerous sport. It's obvious we are doing what we want to do but nothing is worth a day like this."

Pedrosa's Honda teammate Andrea Dovizioso, added: "In such circumstances words are superfluous.

"I'm thinking of his father and his mother. I've also got a child and what's happened today is the worst thing you could imagine. Out on the track we fight hard but misfortune is often just around the corner.

"Mario was a courageous rider and he always fought hard. We've battled on the track since we were kids. I often saw him fall off but without any harm. He appeared indestructible. What's happended here, it seems impossible."

Motor sports have endured a nightmare stretch and Simoncelli's death looked certain to throw up more awkward questions over safety.

Last weekend, 2010 MotoGP champion Jorge Lorenzo crashed during warm-ups in Australia, severing a finger that was later surgically reattached, while two-time Indianapolis 500 winner Dan Wheldon died in a 15-car crash in Las Vegas in Indy Car.

The race at the Sepang circuit was immediately red-flagged and organisers later announced it had been cancelled.

Edwards also fell and suffered a separated shoulder, a race doctor said, while Rossi rode away unscathed.

Rossi, racing for Ducati, left the track devastated at the death of his "good friend", Ducati team manager Vittoriano Gureschi told AFP.

"Valentino is shocked by this incident. Valentino is a strong man but this incident is a big tragedy. He has lost a good friend," he said.

"Marco is a wonderful person. He is friendly with everybody. He loved this sport," Gureschi said.

Sepang is a notoriously tricky circuit due to Malaysia's heat, humidity and frequent rain, but conditions on Sunday, although hot, were fine.

Spain's Pedrosa had seized pole position a day before despite taking a tumble, as did Rossi.

Before the race, Sepang officials had promised special attention on safety following the previous weekend.

"It is a sad back-to-back weekend for motorsports," Sepang Circuit chairman Mokhzani Mahathir told reporters after Simoncelli's death was announced.

"We try our best to avoid incidents and prepare for the worst. You see, 99 percent of the time, riders falling and walking away. Only one percent do not."

Race director Paul Butler promised a thorough investigation.

The last fatal crash in the world motorcycling championship was last year when Japan's Shoya Tomizawa was killed.

Although there is a final race in Spain still to come, Stoner has already secured the 2011 championship with his win last weekend in a highly eventful Australian Grand Prix.

Stoner won at Phillip Island by just two seconds over Simoncelli, whose second-place finish was his best yet in MotoGP.

"You can never guarantee a 100 percent safe race," Mokhzani said.

"You expose yourself to danger when you race. As professionals, they know MotoGP is dangerous. Believe it or not, that is what they live for. Our condolences to Marco. He will be missed dearly."

The flamboyant and well-liked Simoncelli's death sparked grief in Italy, where officials announced sports events would observe a minute's silence.

Italian football giants Inter and AC Milan were among clubs to offer their commiserations.

AC Milan said it "offers a hug to the family of Marco, a huge rossonero fan, and we want to offer the most sincere and heartfelt condolences in this sad moment."