Beijing Olympic officials carried the torch off an Air China plane from St Petersburg, Russia, and were met by Chinese ambassador Fu Ying and minister for the Olympics Tessa Jowell.
About 80 athletes and celebrities will carry the torch by foot, bike, boat and bus, during Sunday's 31-mile relay, starting at Wembley Park and ending in Greenwich.
Anti-China protesters are set to appear at key points along the route but police have warned them they will be dealt with firmly if they try and disrupt the torch's journey.
The event has drawn "substantial" interest from groups who plan to stage protests against China's human rights record and its role in Myanmar (Burma) and Tibet, Scotland Yard has said.
Police have been notified of at least six protests, including one outside Downing Street where Gordon Brown is due to greet the flame, carried by a local schoolboy.
About 2,000 police officers will be on duty during the day, including some on bicycles to keep up with the torch.
Among the groups protesting will be Free Tibet, Falun Gong and the Burma Campaign.
A spokesman for Burma Campaign UK said: "Campaigners will be demanding China stops supporting Burma's brutal military dictatorship."
"If people decide to act outside the law, particularly if they make any incursion towards the torch, they will be dealt with swiftly and appropriately by our officers," police Commander Bob Broadhurst said.
Organisers will be hoping no more torchbearers drop out of the event after a handful pulled out in the run-up to Sunday.
The BBC's deputy director general Mark Byford backed out amid concerns his participation would compromise the corporation's journalistic standards.
Comedian Francesca Martinez also withdrew after telling Channel 4 News she believed there was a conflict in the Olympic ideals of unity and world peace and violence.
China's Ambassador to London, Fu Ying, was expected to take part, despite concern she would act as another focal point of protests.
Tens of thousands of spectators are expected to ignore the controversy and line the route.
Among those they will be cheering on will be tennis player Tim Henman, the girl band Sugababes, actress Denise van Outen, news presenter Trevor McDonald, violinist Vanessa Mae, yachtswoman Ellen MacArthur and Arsenal footballer Theo Walcott.
Former rower Steve Redgrave, who won gold medals at five consecutive Olympics, will start the relay, and it will end when the flame is handed to Kelly Holmes, who won two gold middle-distance medals in the 2004 Games.
Mini-carnivals will be held along the way, including costumed dragons and lions in Chinatown.
The Olympic torch heads to Paris after London.
The route will take in 21 international cities and every Chinese province, before being carried into the Beijing Games' opening ceremony on August 8.
