By the CNN Wire Staff
LONDON (CNN) - The Olympic torch relay began its first full day in London on Saturday, marking the start of the final week-long countdown to the opening of the 2012 Games.
The flame was handed to the day's first torchbearer in Greenwich, home to the Equestrian Arena for the Games.
The torch was also carried up to the roof of the North Greenwich Arena, where gymnastics events and the basketball finals will be staged.
Later in the day, it will reach the Tower Hamlets neighborhood, one of the poorest in London, which has benefited from millions of dollars of investment in infrastructure and regeneration projects as a result of the Games.
The torch made a dramatic arrival at the Tower of London on Friday evening, when a Royal Marine commando, carrying the flame in a lantern descended a rope 180 feet from a Royal Navy Sea King helicopter into the grounds.
London Mayor Boris Johnson was there to greet the torch at the Tower, which dates to the 11th century. The flame spent the night in the safekeeping of the Jewel House, home of the Crown Jewels.
Over the next week, nearly 1,000 torchbearers will carry it along nearly 200 miles of streets around the capital before it takes a starring role at the opening ceremony at the Olympic Park next Friday.
More than 10 million people lined the streets of more than 1,000 communities around Britain to see the torch, which has been carried by more than 7,000 torchbearers over the past two months, Olympic organizers said.
The buildup to the Olympics has been overshadowed by a security fiasco caused by the failure of contractor G4S to supply enough guards for the Games.
But Johnson told CNN's Becky Anderson that while London authorities are not complacent, he is confident in their ability to keep visitors safe.
"We do everything in our power to maximize security at the venues and that's working well now -- the army, the military as well as G4S are providing very safe conditions on the site, plus there is a huge amount of work that nobody has seen involving the intelligence service," the mayor said
"That's something about an event like this -- you listen out for what may or may not be happening on the Internet, you try as hard as you possibly can to monitor the potential suspects."
Johnson said it is impossible to guard against all "unknown unknowns" and that's why strong security is needed at venues.
"At no stage can you guarantee that someone won't do something moronic, stupid, violent and tragic, not just in London but the whole of the UK is a potential target in this period," he said.
But at the same time, he said, "the intelligence is as reassuring as it could be under the circumstances. The threat level as far as we are aware has slightly come down in the last 12 months, but you can never be certain."
Some 3,500 extra military personnel have been called up to meet the shortfall in recruitment by G4S. Another 1,200 have been placed on standby as a contingency, the government said this week.