Sunny skies greeted the runners and a huge crowd of onlookers as the flame was taken from its overnight base in the Tower of London and set off on a meandering tour of the host city for the summer games that begin next week.
Saturday's relay officially started at the Royal Greenwich Observatory and passed by London landmarks such as Queens House and the restored clipper ship Cutty Sark, the BBC said.
Comaneci, who won five gold medals at the 1976 Olympics carried the flame on the roof of the North Greenwich Arena where she handed it off to basketball star John Amaechi.
Other current and former athletes who took part in the relay included triple-jumper Phillip Odowu, gold-medal javelin thrower Tessa Sanderson-White and Fabirce Muamba, the professional soccer player who stunned Britain when he collapsed in the middle of a match this spring of a heart attack and nearly died.
Also in the course was 101-year-old marathon runner Fauja Singh, round-the-world sailor Sir Robin Knox-Johnston and ex-paratrooper Jaco Van Gass, who lost an arm in Afghanistan.
The entertainment world was represented by pop stars Dizee Rascal and Paloma Faith, who the BBC said carried the torch in a tailored uniform and red high heels.
The torch will have been carried 8,000 miles by 8,000 runners over 70 days when it arrives at the opening ceremony July 27, the BBC said.