LONDON - London began the final year countdown to the 2012 Olympics with a splash of water and a flash of gold on Wednesday as IOC president Jacques Rogge formally invited the world and its athletes to the Games next July.

British Olympic diver Tom Daley prepares to perform a dive into the dive pool in the London 2012 Olympics Aquatics Centre in east London July 27, 2011.??[Photo/Agencies]

British medal hope Tom Daley made the first dive into the chilly waters of the newly-opened Aquatics Centre before, in Trafalgar Square, London 2012 chairman Sebastian Coe and Britain's Princess Anne unveiled the medals all will be striving for.

Chunky and bold, the medals will be larger than those in many previous Games and heavier too.

Addressing the crowd in the same square and on the same spot where Londoners heard in 2005 that they had won the right to be hosts of the Games for an unprecedented third time, Prime Minister David Cameron said the city would be ready.

"Six years on, with just one year to go, the sense of excitement is even greater today than it was then," he declared.

"I think this has the makings of a great British success story."

International Olympic Committee chief Rogge, speaking at 20.12 European time, called on the world's finest athletes to attend the Games of the XXX Olympiad in "the nation that invented modern sport and the concept of fair play."

"The athletes will be ready, and so will London," said Rogge, who earlier had assured reporters that London's preparations were on a par with great Games of the past.

"I came into the Aquatics Centre and I had a visual shock. I have seen so many venues in my life, and this is unique," he said, standing under a roof whose wooden underside resembles the vast belly of a whale.

"The Olympic Stadium, the Velodrome, the Aquatics Centre...a great job done by London," added Rogge.

"You can't compare venues that are built in different times and different countries for different populations. But I would say in terms of operational readiness, London is on a par with Sydney 2000 and Beijing 2008. And these were two fantastic Games so this bodes well for London."

Coe, himself a double 1,500 metres track gold medallist, said the one year to go milestone was a big one.

"With a year to go, we are inviting the athletes, spectators and visitors from around the world to come to the UK next summer," he declared. "It's 'London Calling'."????

Big Moment

Coe, who kicked off the day by having his footprints cast in clay at the St Pancras Station where many visitors will arrive from the continent, said there was still much to do but was confident that what was within the organisers' control was under control.

London will be the first city to host the Olympics for a third time, after previously doing so in 1948 and 1908, and faces considerable challenges ahead with a creaking transport system and the biggest peacetime security operation the country has ever seen.

The completion of the Aquatics Centre at least allowed organisers to congratulate themselves on delivering the Olympic Park's permanent venues with 12 months to go, largely on time and on budget.

London mayor Boris Johnson, standing next to a clock declaring 366 days to go due to 2012 being a leap year, invited the world to a party like no other.

"Nothing and no-one is going to stop us in our work of preparing London for the greatest event that has taken place in this city for the last 50 years," he said.

"The Olympic venues are already so ready that we might as well call a snap Olympics tomorrow and catch the rest of the world napping," he added in a typically rumbustious speech that has Rogge smiling in appreciation.

More than 3.5 million tickets have been sold so far, with all sports bar soccer sold out in the British offering, and a quarter of a million people have applied for 70,000 volunteer positions.

Some 15,000 athletes from more than 200 countries will compete at the Olympic and Paralympic Games, with some 10,000 team officials and 20,000 accredited media also expected.

Britain, fourth in the gold medal table in Beijing in 2008, will have 550 athletes competing across the 26 sports and chasing an even greater tally.

"There is much to do in the next 12 months but we can take huge comfort in the progress that has been made so far," said Coe.

"We are absolutely on track and determined to stage Olympic and Paralympic Games which will deliver on the promises we made in Singapore (in 2005), inspire the athletes and make the nation proud." ?(Reporting by Alan Baldwin, editing by Justin Palmer; For Reuters sports blog Left Field go to: http://blogs.reuters.com/sport) ?