Usain Bolt and Michael Phelps defend their hero status against new rivals when the Olympics return to the cradle of modern sport for the London Games.
The Jamaican Bolt redefined the men's sprint with gold medal runs in world record times in the 100m, 200m and 4x100m at the 2008 Olympics in Beijing. He has further improved the records since then but also has a formidable rival in countryman Yohan Blake.
Phelps won an unprecedented eight gold medals at one Games four years ago for an overall record 14, and the American swimmer also has a major challenger from his own camp in Ryan Lochte.
As in the past, athletics and swimming will take centre stage when London hosts the Olympics for the third time from July 27 until August 12, following 1908 and 1948.
A new Olympic Park in Stratford is the heart of the Games but iconic venues such as Wimbledon (tennis), Wembley (football) and Lord's (archery) are also part of the show.
Britain has spent an overall STG9.3 billion ($A14.3 billion) on the Games despite a difficult economic climate, with security taking up a huge chunk of money in a city where suicide bombers killed more than 50 people the day after London were elected host city by the International Olympic Committee in July 2005.
A last-minute glitch when a security company failed to provide the promised number of personnel was swiftly handled by the government which will provide additional troops to safeguard the Games.
London's transportation system is facing its biggest test to bring millions of visitors, officials and media, to the venues where some 10,500 athletes from 204 countries will be fighting for 302 gold medals on offer in the world's biggest sports event.
London will not be able to duplicate the organisation of Beijing 2008 as it lacks the human resources of communist China, and IOC chief Jacques Rogge said the British to look at their own proud sports history to give their Games an identity.
"I always said this is the country that invented modern sport in the second half of 19th century. This is also the country that put sport into the school curriculum and use sport as a tool of education. They were the first to do that. This is a country that loves sport," the IOC president said.
And just as Rogge is a former Olympian as a yachtsman for Belgium, London 2012 has an ex-athlete at the top of its organising committee LOCOG in the form of of two-times 1500m gold medallist Sebastian Coe.
"Seeing the Games through the eyes of a competitor is really an important aspect. That's not just an emotional connection, it actually makes the project a better project. If you are driven to deliver for the athletes you are probably going to deliver a pretty good Games," Coe said.
"I don't want to let the athletes down. I want every competitor to look at me and say 'you did everything you could possibly have done to make my experience both social and competitive the best experience I have ever had at a sporting event.' That for me is what drives me."
Bolt and Phelps will be the top attraction of the competition although a 526-plus strong British home team also aims to make a big splash after coming a surprise fourth in the medal table in Beijing.
Teenager Missy Franklin is a new US swim star while 38-year-old Ryan Giggs is to shine in football where Team GB - unlike in regular events - is made up of players from England, Wales, Scotland and Northern Ireland.
David Beckham was not picked for the British football team, but Kevin Durant and Kobe Bryant lead the latest US basketball dream team, Roger Federer will be seeking a first tennis gold a few weeks after his seventh Wimbledon crown and British sailor Ben Ainslie seeks a fourth gold.
China garnered the most golds at its home Games four years ago and the world's most populous country is expected to battle out the top of the medal table again with the United States, who had the most medals overall in Beijing.
Women from Saudi Arabia will be competing for the first time on the Olympic stage for full gender equality among all 203 expected participating nations.
More than 6,000 doping tests are to ensure fair competition on the playing fields and act as a deterrent for potential cheaters. Like in the past, all samples will be kept for eight years for possible retesting.