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.
Four-time Olympic rowing gold medallist Matthew Pinsent runs through Hampton Court Palace in London with the Olympic Torch, July 27, 2012.
This photo provided by LOCOG shows the royal barge Gloriana carrying Olympic flame burning in the cauldron as it makes its way down the River Thames towards Richmond Bridge during the final day of the Olympic torch relay in London, July 27, 2012.
In this photo provided by LOCOG, British actresses Joanna Lumley, center right, and Jennifer Saunders, center left, carry the Olympic Flame between Lambeth and Kensington and Chelsea, in London, July 26, 2012.
This photo provided by LOCOG shows torchbearer 170 passing the Olympic Flame to Torchbearer 171 in front of Buckingham Palace in the presence of the Duke and Duchess of Cambridge and Prince Harry, July 26, 2012.
Torchbearer Tyler Rix lights the Olympic Cauldron during the Olympic torch relay celebrations in Hyde Park, London, July 26, 2012.
Britain's Prime Minister David Cameron (2nd L) and his wife Samantha (L) look on as Olympic torch bearer Florence Rowe (R) receives the flame from Kate Nesbitt (3rd L) in Downing Street, July 26, 2012.
Bollywood actor Amitabh Bachchan carries the Olympic flame on the torch relay leg between The City of London and the borough of Southwark in London, July 25, 2012. (Photo provided by LOCOG)
Venus Williams of the U.S. waits with the Olympic Torch on Murray Mound (also known as Henman Hill) at the All England Lawn Tennis Club before the start of the London 2012 Olympic Games in London July 23, 2012.
Former England striker Michael Owen poses with the Olympic Torch at Battersea dogs home in London, July 23, 2012.
Torch bearer Amelia Hempleman-Adams, 17, stands on top of a capsule on the London Eye as part of the torch relay ahead of the London 2012 Olympic Games in London July 22, 2012.
Former world heavyweight boxing champion Lennox Lewis carries the Olympic torch through Danson park in Bexley, south east London as part of the torch relay ahead of the London 2012 Olympic Games July 22, 2012.
British around-the-world sailor Sir Robin Knox-Johnson carries the Olympic Torch around the restored Cutty Sark boat at Greenwich in London July 21, 2012.
Former U.S. Olympic athlete Michael Johnson holds the Olympic Torch at Stonehenge, a World Heritage site, in Salisbury, southern England July 12, 2012.
Roger Bannister (L) passes the Olympic flame to torchbearer Nicola Byrom during the London 2012 Olympic Games torch relay at the Iffley Road Stadium in Oxford, southern England July 10, 2012.
Britain's Queen Elizabeth (R), Prince Philip (2nd R), and Olympic torch bearer Gina Macgregor (2nd L) watch as torchbearer Phil Wells takes the Olympic Flame at Windsor Castle, west of London July 10, 2012.