Mellouli won the 1,500 metres freestyle gold at the 2008 Beijing Olympics and used his superior speed to burst clear of his rivals and win the gruelling 10 kilometre race at the Serpentine in London's Hyde Park.
Germany's Thomas Lurz won the silver medal and Richard Weinberger of Canada the bronze but no one could keep up with the 28-year-old Mellouli after he charged into the lead approaching the end of the fifth of the six loops.
He quickly opened up a three body length lead over the chasing pack and maintained his advantage to the end, slapping the touch pads to win gold in one hour 49 minutes and 55.1 seconds.
Lurz was 3.4 seconds behind in second with Weinberger a further 1.8 seconds behind as the top 14 finishers completed the exhausting event within a minute of each other.
The race was contested at a recreational lake in Hyde Park that was constructed in 1730 and remains a London tourist destinations.
A massive crowd lined the banks to watch the race, which was only introduced into the Olympic programme at Beijing four years ago.
Mellouli won the world championship for 800 metres in 2007 but was stripped of his title and banned from competing for 18 months after testing positive to amphetamines at a meet in the U.S. in late 2006.
Mellouli, a student at the University of Southern California at the time of the offence, said he had taken an Adderall pill two days before he tested positive so he could stay awake to finish a university assignment.
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