The Princess Royal has unveiled the London 2012 gold medals as the one-year countdown to the Olympics began.
The medals depict the Greek goddess of victory, Nike, stepping out of the depiction of the Panathenaic to arrive in the host city.
Thousands gathered in Trafalgar Square where David Cameron promised London's would be the "greatest games".
Meanwhile, diver Tom Daley performed the first official dive at the Aquatics Centre, the latest venue completed.
Synchronised swimmers formed the number one for London 2012 chairman Lord Coe and International Olympic Committee (IOC) president Jacques Rogge.
At Trafalgar Square, Mr Rogge and London Mayor Boris Johnson invited the world to come to the British capital to celebrate the Games next summer.
Mr Johnson boasted that London is already so ready for the 2012 Games that "we might as well call a snap Olympics tomorrow".
In other events:
- Lord Coe, Olympics Minister Hugh Robertson and former Olympic silver medallist Colin Jackson have made plaster casts of their feet at London's St Pancras Eurostar terminal to mark the "first steps" toward the 2012 Games
- The Foreign Office has organised a range of events at nearly 100 of its posts across the world to encourage visitors, businesses, students and sports people to get involved
- The British Mission at the United Nations in New York is holding an event featuring nine-time Olympic gold medallist Carl Lewis and the UN's Secretary General Ban Ki Moon
- The Royal Mail has announced it will issue special gold medal stamps to celebrate all Team GB Olympic golds
- London 2012 will be the first summer Games to use "biological passports" to try to stop drug cheats
- Lord Coe said the organisers' plans were on schedule and that the day was a "big moment" for the London 2012 Organising Committee (Locog)
- The lack of capacity would be the main problem on London's travel network says BBC London's transport correspondent Tom Edwards who says Transport for London (TfL) is urging commuters to work from home or use different routes
Mr Cameron declared the Trafalgar Square event a "great night for London, a great night for Britain".
"This has the makings of a great British success story. With a year to go, it's on time, it's on budget," he said.
"It's about great sport, with great British athletes like Tom Daley and Chris Hoy taking part."
Mr Rogge said the world's finest athletes were dreaming of competing in "this magnificent city".
"As in 1908 and 1948, they knew they would be coming to the nation that invented modern sport and the concept of fair play," he said.
With a year to go, the Olympic Delivery Authority (ODA) said construction had finished on all six of the main permanent Olympic Park venues.
The £269m Aquatics Centre was designed by Zaha Hadid with the aim of being the "Gateway to the Games".
BBC sports editor David Bond said he had been "reliably told" the centre was only finished six days ago.
After double Commonwealth gold medallist Tom Daley performed the first dive in front of several hundred fans, he said the pool looked "incredible".
"The atmosphere was great. The amount of cheers and stuff. It's going to be awesome next year," he added.
John Armitt, chairman of the ODA, said the project had created jobs and training at a critical time for the UK economy with about 40,000 people working on the project to date, and 98% (£6bn) of the building contracts going direct to UK businesses.
As the final countdown to the Olympics gets under way, the Metropolitan Police is facing challenges as it gears up for the biggest peacetime security operation in its history.
Two senior police officers, including Commissioner Sir Paul Stephenson, resigned this month over the News of the World phone-hacking scandal, but Lord Coe says security plans for London are "robust".
The force itself says it is "business as usual" and the Met branch of the Police Federation has said although there is low morale it had more to do with major reform and cuts in police budgets than losing their leader.