London 2012 began its final year countdown on Wednesday with swimmers making a splash in the newly-opened Olympic pool and IOC president Jacques Rogge putting preparations on a par with great Games of the past.

British medal hope Tom Daley was due to make the first dive later in the evening, before Rogge formally invites the world and its athletes to come and join the party next July.

"With a year to go, we are inviting the athletes, spectators and visitors from around the world to come to the UK next summer," declared London 2012 chairman Sebastian Coe.

"It's 'London Calling'."

International Olympic Committee head Rogge, speaking at the Aquatics Center beneath the undulating wave-shaped roof whose underside resembles the vast belly of a whale, was suitably impressed.

"I came into the Aquatics Center and I had a visual shock. I have seen so many venues in my life, and this is unique," he declared after watching synchronized swimmers and local teenagers test the water for the first time.

"The Olympic Stadium, the Velodrome, the Aquatics Center...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."

BIG MOMENT

Coe, who kicked off the day by having his footprints cast in clay at St Pancras Station, said entering the final stretch was a "pretty big moment for us."

London won the bid for the Games in 2005, and will be the first city to host the Olympics for a third time after previously doing so in 1948 and 1908.

The completion of the Aquatics Center allowed organizers to congratulate themselves on delivering the Olympic Park's permanent venues with 12 months to go, on time and on budget.

"To have all... permanent venues complete with a year still to go to the Games is a great achievement, and a firm sign that we are well on track to deliver a truly spectacular show in 2012," said London Mayor Boris Johnson.

Rogge will formally invite the world's athletes to the Games at an evening event -- at 20.12 European time -- in Trafalgar Square.

Before that, medal hope Daley will make the first dive into the pool at the Zaha Hadid-designed Aquatics Center.

"One year to go is a special time for any host nation," said Rogge in a statement.

"It is the moment... when Olympic dreams start to come into focus and when the world turns its attention in earnest to the city that will welcome it in only 365 days' time."

Johnson will also welcome the world while the design of the medals, an eagerly awaited secret, will be unveiled for the first time.

"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."

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.

"Knowing full well that we have gone from being the hunters to being the hunted, no detail is being overlooked and absolutely nothing is being left to chance," said British Olympic Association (BOA) chairman Colin Moynihan.