Battle-hardened swimmers clashed, splashed, tugged and jostled as the Olympic Games showcased its first open water marathon yesterday with a race few of the athletes will forget.

Russian Larisa Ilchenko won a two-hour tussle that saw swimmers fighting off elbows, buoys and stray water bottles for a chance to win the gruelling discipline's first Olympic medals.

Ilchenko, an eight-times world champion, said open water swimming was about the survival of the fittest.

"It's difficult, I had to clash on numerous occasions," Ilchenko told reporters.

"I was trying to break away from the Brazilians, they were quite aggressive, sometimes bordering on unsportsmanlike. This is swimming after all, not boxing."

Australia's Grant Hackett, who dominated distance freestyle in the pool for close to a decade, failed in his bold attempt to qualify for the Olympic "water wrestling", falling foul of dirty tricks and aggressive manoeuvres.

"Even if you're a superstar, it doesn't mean anything when it comes to open water," Ilchenko said of Hackett.

The race got under way when 25 swimmers leapt off the starting platform and charged for a spot in the lead pack, with clashing hands, tangled arms and the grim prospect of a foot in the face and a black eye.

Bronze medallist Cassandra Patten, who juggles open water with the civility of traditional Olympic swimming events, said the race was one of the cleanest she had seen.

"For me it was an easy race, there was no argy-bargy," said the Briton, who led the race before Ilchenko's stunning final-minute surge.

"I wanted to stay out of trouble. You're going to be clashing arms and knocking each other.

"It's what happens, it's all part of the race."