(Clarifying in 10th para that it is the first time a city has received the Olympic flame twice for a summer Games)
* Sun ignites London Games torch
* Torch will land in Britain on May 18
By Karolos Grohmann and Deepa Babington
ANCIENT OLYMPIA, Greece, May 10 (Reuters) - The countdown to the London Olympics began with the kindling of the Games torch by the sun's rays in ancient Olympia on Thursday, sparking a relay that will culminate with the lighting of the Olympic stadium's cauldron at the opening ceremony on July 27.
On a warm and sunny day at the site of the ancient Olympics, actress Ino Menegaki, playing the high priestess, appealed to sun God Apollo and needed only a few seconds to ignite the torch at the ruins of the Temple of Hera with the help of a parabolic mirror, 78 days before the Games get under way.
The relay's first torchbearer, Spyros Gianniotis, a Liverpool-born Greek swimmer who won the gold medal in the 10-km open water event at the 2011 world championships, started the seven-day Greek leg of the relay before the flame is handed over to London organisers on May 17 and flown to Britain a day later.
The second torchbearer was 19-year-old Alexander Loukos, a Briton of Greek origin.
"With this ceremony we begin the final countdown to a dream that came to life seven years ago in Singapore, when London was selected to host the 2012 Games," said International Olympic Committee (IOC) President Jacques Rogge in a brief speech.
This is the last torch ceremony of his presidency, as Rogge steps down next year after 12 years in charge.
"The energy that passes from the sun to the Olympic flame will light a torch that will travel from this birthplace of the ancient Olympic Games to the country that invented modern sport and the spirit of fair play, heralding the opening of the 2012 London Games on July 27," the Belgian surgeon said.
Olympic, London and Greek officials were seated inside the stadium where the ancient Games were held and some 5,000 spectators watched the ceremony from the grassy slopes hugging the arena.
A strong police presence around the ancient site, and in the town of Olympia, made sure the event went off without a hitch. Four years ago human rights activists briefly disrupted the Beijing Olympics ceremony.
London becomes the only city to have received the Olympic flame twice for a summer Games - the first time being for the 1948 event.
Britain's capital also hosted the 1908 Games, but the torch-lighting ceremony and relay were introduced for the first time for the 1936 Berlin Olympics as part of Nazi propaganda.
"I feel incredibly excited and I think it's a very, very big moment," London Games chief Sebastian Coe told Reuters.
Coe, twice a 1,500 metres Olympic gold medallist, visited Olympia in 1975 as an 18-year tourist, hardly expecting to be back in 37 years to watch the torch being lit for an Olympics hosted by Britain.
"For me, it really links what I did at Los Angeles and Moscow (Games) with the ancient Games. I think for me this is probably the moment that what I did in '80 and '84 properly comes into context," he said.
The 70-day British leg of the relay will use 8,000 torchbearers and travel 12,800 km around the country, taking in 1,018 communities and the 1,085-metre summit of Snowdon, before ending inside the Olympic stadium on the opening day of the Games.
The relay will also take in landmarks around Britain with the flame travelling by canal boat, cable car, tram, steam train, hot air balloon and even motorcycle sidecar on the Isle of Man TT course.
More than 95 percent of the population will be within an hour of the route.
"As a country we have a tradition of coming to big events like this quite late and then we get very, very enthusiastic about it," Britain's Sports and Olympics minister Hugh Robertson told reporters.
"I've been sort of expecting that there would be a slow, steady buildup to this and as soon as the torch arrives I suspect, if not hysteria, quite enormous enthusiasm is going to take over." (Editing by Clare Fallon)