TOKYO, July 26 (Reuters) - The men"s Olympic triathlon suffered a rare and embarrassing false start on Monday as half the field dived in while the others were blocked off by a boat, leading to some frantic action from associated craft to get the message over to the initially unaware swimmers. Fifty-six men lined up on a pontoon in Tokyo Bay for the 06.30 local start but as the starter sent them away for the opening 1,500 metre swim leg, around a third found their way blocked by a camera boat. With those left on the pontoon stranded, a mini-flotilla of boats and jet skis flew into action to head off the group, 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 200 metre warm up than they had expected. The race got underway safely around 10 minutes later.