By Martyn Herman

RIO DE JANEIRO (Reuters) - Belgium's Greg Van Avermaet snatched gold in a thrilling finish to the men's Olympic road race, out-sprinting Denmark's Jakob Fuglsang by the Copacabana after Vincenzo Nibali's hopes ended in a high-speed crash on Saturday.

Nibali was surging toward victory following a late attack after six hours of racing around Rio's exotic coastline when he careered out of control on the final descent.

A 237.5km course, taking in 5,000m of tough climbing through lush green hills next to the rolling Atlantic, promised unpredictability and it delivered in spades as nearly half of the 144 starters failed to finish.

Italian Nibali was taken to hospital with a possible fractured collarbone.

After mechanical mayhem on several loops of the hilly and partly-cobbled Grumari section the fireworks began in earnest on the Vista Chinesa circuit featuring a steep 8.9km climb the peloton tackled three times.

It was predicted the race would be won there, but it was where Nibali lost it.

He and fellow Italian Fabio Aru shredded the race with an audacious attack on the final ascent and Nibali looked poised to add an Olympic title to his Tour de France, Giro and Vuelta victories when calamity struck.

Accelerating downhill with Colombia's Sergio Henao and Poland's Rafal Majka for company, Nibali hit the tarmac hard, as did Henao, leaving Majka alone with 12km to go.

Majka weaved between the wreckage and looked set to win Poland's first Olympic cycling gold but Van Avermaet, who briefly wore the yellow jersey at the Tour de France last month, and Fuglsang had other ideas.

SPRINTING PROWESS

They caught Majka with little more than a kilometer left and Van Avermaet used his sprinting prowess to pedal to glory.

Majka hung on for the bronze five seconds back.

A course that weaved through Rio's verdant hills produced a race so unpredictable that even Van Avermaet, who suffered an early puncture, was not sure what Nibali's crash meant.

"When I saw the crash I was confused about how many riders were still up the road," Van Avermaet, winner of Belgium's first Olympic gold medal since 2008, told reporters.

"I was working with Fuglsang. We saw Majka. We knew it was possible. I am so happy for gold. Everyone said all week it was for everyone else."

It was a disappointing day for Tour de France champion Chris Froome who finished 12th after struggling on the final climb and having to change his bike early on.

Fuglsang said he had been extra vigilant on the descents in what he described as a "crazy race".

"Maybe guys took too many risks," he said. "I decided to stay safe and come down in one piece."

Several other medal contenders crashed. Australia's Richie Porte slammed into a wall and was lucky to avoid serious injury while Froome's British team mate Geraint Thomas came off later before remounting to finish 11th.

Poland's Michal Kwiatkowski produced a courageous ride, spending most of the day in a breakaway and then as lone leader before being caught with 45km to go.

He was still in the hunt before cramp got the better of him.

Even then it looked as though Majka could complete the job for Poland, only for the 26-year-old to fall agonizingly short.

"In the end I had such hard cramps in my legs I could barely pedal," he said. "When the others caught me I knew that it was impossible to win."

(Editing by Ed Osmond)