Beijing - Michael Phelps on Sunday became the most successful athlete at an Olympic Games when he won his eighth gold medal as the US claimed the men's 
4x100 relay medley in world record time.

On the final day of swimming at the Beijing Games there were also gold medals for Germany's Britta Steffen, Tunisian Oussama Mellouli and the Australian women's 4x100 medley relay team.

Steffen ensured herself the swimming sprint double, winning in Olympic record time of 24.06 seconds, a day after striking gold in the 100m freestyle.

Silver went to 41-year-old Dara Torres of the US in a time of 24.07, while Australian Cate Campbell took bronze in a time of 24.17.

The German gave much of the credit for her performance to her psychologist.

"I really worked a lot with my psychologist. When I was on the blocks this time I didn't feel like everyone else was better than me. I know now that this is rubbish, I can do this," she said.

In the next race Tunisian Oussama Mellouli denied Australian Grant Hackett a place in the history books as he won the Olympic gold medal in the men's 1,500m freestyle in a time of 14 minutes 40.84 seconds.

Hackett was attempting to become the first swimmer to win the 1,500m three times in a row. He won silver in 14:41.53, while Canadian Ryan Cochrane took bronze in 
14:42.69.

Mellouli only returned from a lengthy doping ban earlier this year after having lost the gold and silver medal he won at the world championships in Melbourne in 2007.

The Australian women's 4x100m medley relay team shaved 3.05 seconds off their own world record when they won in 3:52.69. The US took silver in 3:53.30, while bronze went to China in 3:56.11.

In the final race of the Olympic swimming competition, the US men's 4x100m medley team won in a world record time of 3:29.34, beating their own best mark from 2004 by 
1.34 seconds.

Silver went to Australia in 3:30.04, who were also under the old world record, while Japan took bronze in 3:31.18.

It was the 25th world record to fall in the Water Cube.

Phelps said he could not really describe what it meant to win eight medals. "I'm lost for words. My team-mates were amazing. These guys made it possible. The relay really made it possible. It shows how much teamwork and togetherness we have. It's amazing to be a part of it." (dpa)