London's famous Big Ben chimed 40 times over three minutes Friday, joined by church bells and car horns to welcome the start of the 2012 Olympic Games
The giant bell, housed in a clock tower overlooking the Houses of Parliament, began its toll at 8:12 a.m. Entertainment preceding the opening ceremony is scheduled to begin at 8:12 p.m. -- symbolically, 20:12.
The Olympic torch was rowed down the Thames river on the royal barge Gloriana
on the final leg of its journey to London's Olympic Park.
The torch relay ends Friday evening with the lighting of the Olympic cauldron during the Games' opening ceremony.
London is welcoming around 10,000 athletes, hundreds of thousands of visitors and a worldwide television audience for the extravagant ceremony. It will be directed by Danny Boyle, the director of the Oscar-winning movie Slumdog Millionaire
, and will feature a performance by British music legend Paul McCartney.
Britain's Queen Elizabeth will officially open the Games. The competition in 26 sports runs until August 12.
Organizers have dealt with a series of last-minute problems, including scrambling to deploy enough security personnel after a private contractor reported staffing problems.
On the eve of the opening, British Prime Minister David Cameron said his biggest concern is a "safe and secure" Olympics.
About 36,000 troops, police and hired contractors will guard the Olympic venues and the streets of London and other British cities. After the private security firm G4S acknowledged earlier this month that it would not be able to hire all of the 10,000 contractors it had agreed to provide, British officials called up additional military personnel to fill the gap.