London began the final countdown Friday to the opening ceremony that will officially kick off its Olympic Games, with some 120 world leaders arriving in the British capital for the spectacle.
Organisers expect more than a billion people worldwide to watch the event on television, while tens of thousands will see the show live at the Olympic Stadium in east London.
The Olympic torch was making its way down the River Thames onboard the Gloriana, the royal barge of Queen Elizabeth II, on the last day of its 10-week journey around Britain.
In Westminster, Big Ben chimed for three minutes from 8:12 am (0712 GMT) to ring in the Games, 12 hours ahead of pre-ceremony entertainment scheduled to start at the symbolic time of 20:12 pm.
The huge bell at the British parliament was joined by hundreds of bell-ringers across the country in a cacophony of ringing to mark the beginning of the extravaganza.
Footballer David Beckham is expected to be among the final torchbearers carrying the Olympic torch as it finally arrives at the stadium for the opening ceremony.
The former England captain, who played a key part in London's original bid for the Games seven years ago, said the thought of the spectacle gave him "goosebumps".
"I have been lucky enough to have been involved in some amazing sporting moments in my career, but nothing can compare with what is about to take place in London," he wrote in The Sun newspaper.
"I know every part of the Games is going to be spectacular and tonight is the night it all kicks off, with the whole world watching Great Britain and the opening ceremony.
"It will be one of those moments people will always remember watching."
Sneak previews of the £27 million ($42 million, 35 million euros) opening ceremony -- filmed at Wednesday's final rehearsal -- suggest it will be a grand but quirky production, reflecting the philosophy of director Danny Boyle.
The Slumdog Millionaire Oscar-winner has promised to create a "picture of us as a nation" and revealed that the eccentric show will feature live sheep and dancing surgeons from the National Health Service.
Audience members at rehearsals for the ceremony have been full of praise for the show, but they have stayed tight-lipped about details.
The weather is expected to be dry for the ceremony after showers during the day in London.
Thousands of VIPs including some 120 national leaders are in town for the ceremony, with guests including Angelina Jolie, US First Lady Michelle Obama and the king of Swaziland.
The Games will see Britain launch its biggest-ever peacetime security operation, and British Prime Minister David Cameron stressed Thursday that safety would be his top priority.
"I think we've made as many preparations as we can. I think we have very good contingency plans in place," he told a press conference at the Olympic Park.
A force of more than 40,000 military and civilian personnel, backed by a huge intelligence operation, has turned the British capital into a fortress to protect venues, athletes and millions of visitors.
But security has been under the spotlight after the government was forced to deploy an extra 4,700 troops to guard the Games because of a recruitment shortfall at private contractor G4S.