Japanese wrestler Tatsuhiro Yonemitsu won gold in the 66kg freestyle with a 3-1 win over India's Sushil Kumar.
Former Commonwealth Games wrestler Non Evans:
"Yonemitsu deserved that. He was the best wrestler from start to finish and deserves the gold medal. He was head and shoulders above Kumar and in total control from start to finish."
The victory for last year's World Championships silver medallist was Japan's first men's wrestling gold since flyweight Mitsuru Sato at the Seoul Games in 1988.
Cuban Livan Lopez and Kazakhstan's Akzhurek Tanatarov won bronze medals.
Kumar, a railway worker from Delhi, was looking to become India's first ever Olympic wrestling champion.
The Japanese wrestler was 1-0 up after a tight first period but pulled clear in the second when he picked up and slammed Kumar, who was accused of biting in his semi-final victory against Tanatarov.
Japan won three of the four women's categories but Yonemitsu delivered the first men's gold.
Egyptian Abdou Omar Abdou Ahmed was
not allowed to take part in qualifying
after arriving at the ExCeL half an hour late.