TOKYO (REUTERS) - The men's Olympic triathlon suffered a rare and embarrassing false start on Monday (July 26) as half the field dived in while the others were blocked by a media boat, leading to frantic action by a mini-flotilla of boats and jet skis to haul back swimmers.

Fifty-six men lined up on a pontoon in Tokyo Bay for the 6.30am (5.30am Singapore time) start but as the starter sent them away for the opening 1,500m swim leg, around a third found their way blocked by a camera boat.

Those left stranded on the pontoon watched as a mini-flotilla headed off those that had started swimming, with around half of them still ploughing on regardless despite the alert horn sounding repeatedly.

Eventually, two jet skis combined to stop the leaders, who returned slowly to the start having undergone a more vigorous 200m warm-up than they had expected.

After declaring an "invalid start" with no blame attached to any competitors, the race got under way safely around 10 minutes later.

Race winner Kristian Blummenfelt said the chaos had not been a major problem.

"I saw the boat and found it quite strange," the Norwegian said. "When I dived in I thought it would most likely be pulled back so I just kept to the left and went steady and tried to look at it as a positive, an extra warm-up."

New Zealand's bronze medallist Hayden Wilde was also phlegmatic - eventually.

"I thought 'I've had a blinder of a start here', then realised that it was because hardly anyone else was around me and I felt pretty gutted," he said.

"But then I tried to make a positive out of it - it was actually good to stretch the arms a bit after hanging around for a bit for 25 minutes before the start."

Not everyone came out of it so well, however. Australian Jake Birtwhistle was kicked during the initial melee and suffered a broken nose, though he still went on to finish 16th.

"It was one of the roughest swims I have been in," he told The Sydney Morning Herald.

"It was a foot, it might even have been in the false start so it was all for nothing as well."

Teammate Aaron Royle, who finished 26th, said: "I just had to laugh to myself as I was swimming back to the pontoon, to think of all the races for that to happen it had to be the Olympics.

"I thought 'well they're not going to start the race because the boat is literally there' and next minute... I guess, there was no communication between the starter and whoever was on the pontoon."

A statement for international governing body World Triathlon did not shed a great deal of light on how the mix-up came about.

"At the start, some of the athletes were blocked by an (Olympic Broadcast Services) boat and as a result we had to start again the race due to an invalid start."