Christine Ohuruogu, centre, crosses the line as the Olympic champion [AFP]

The 24-year-old Londoner powered home over the last 50 metres to cross the line first in 49.62 seconds and add the Olympic title to the world title she won in Japan last year.

"Everyone always dreams about winning but you never think there's a reality to something you dream about," Ohuruogu said.

"I crossed the line and was thinking, 'Oh my gosh, I've won...I'm just so proud of myself'."

Shericka Williams of Jamaica finished strongly to take the silver in 49.69 and American Sanya Richards faded badly after leading into the straight and had to settle for bronze in 49.93.

Doping ban

Ohuruogu looked like missing out on Beijing when the British Olympic Association banned her life from competing at the Games after she missed three out-of-competition doping tests in 2006.

However, she lodged an appeal against the life ban because she had already served a 12-month suspension and was cleared to compete at the Olympics.

"My coach has always told me you have to go in expecting to win even if you have a bad day or you're not the best in the field," she said.

"He believes in me so much, enough to say, 'Chris go out and win each round because why would you want to go to the next one knowing that someone's beaten you?'"

Richards went into the race as favourite but failed in her bid to win her first major international title.

The 23-year-old American won relay gold at the last Olympics and silver in the 400 at the 2005 world championships before she was struck down by Behcets syndrome, a rare disease that drained her energy and left her unable to speak after her mouth filled with ulcers.