China's Sun Yang destroyed the rest of the field to break his own world 1500m record and win his second gold medal of London 2012.
Sun finished in a time of 14 minutes 31.02 seconds - 3.12 seconds faster than the world record he set at last year's World Championships in Shanghai.
Sun was eight seconds clear of Canada's Ryan Cochrane with defending champion Oussama Mellouli of Tunisia in third.
What a great swim by Sun. He swam the last 100m in 53.5 seconds which is superb. After the shaky start he had he reset himself and I think it shook him for the first length. But you have seen the master of distance swimming in action.
Britain's Dan Fogg finished back in eighth in 15:00.76.
Sun had won the 400m freestyle
joint second in the 200m event
earlier in the Games.
There was drama before the race as Sun dived into the water before the starter's gun, after apparently hearing a noise in the crowd, while everyone else remained on the blocks.
Sun looked at the starter, got back out of the pool and waited to see if he would be disqualified. But he was relieved when the starter allowed him to go.
"I could not hear the judge because there was noise in the venue," he said afterwards through an interpreter. "I thought I was going to be disqualified. I have done well because I was in very good shape. I really wanted this gold medal."
Once the action got underway, the 20-year-old, who had broken Grant Hackett's long-standing world mark last year, set off quickly and was above world-record pace from the start.
He quickly put distance between himself and his rivals and from then on it was a race to see by how much he would break his old record.
Behind him, Cochrane and Mellouli got involved in a tussle for medals after the challenge of South Korea's Taehwan Park faded.
Cochrane eventually emerged victorious by just over half a second but Mellouli will get another chance for gold when he races in Friday's 10km open water event in Hyde Park.