The Olympic flame has embarked on a 70-day tour of the UK in the run-up to the London 2012 Games.
Triple Olympic gold medallist sailor Ben Ainslie was the first torchbearer in the 8,000 mile relay, which left Land's End, Cornwall, at 07:15 BST.
Cheering crowds greeted the flame as it passed through lanes and streets.
The flame is travelling through Cornwall into Devon where it ends its first day with an evening celebration on Plymouth Hoe.
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London 2012 Olympic torch relay
It is being carried by members of the public, young and old, who were nominated for their achievements and community work.
Each of the 8,000 torchbearers will run for about 300m with the flame before lighting the next bearer's torch in a "kiss".
The BBC's Jon Kay in Marazion, Cornwall, said the numbers of people turning out to watch the relay had been "quite astonishing".
There was "a great sense of spirit from tens of thousands of people," he said, adding that the high turnout was slowing the torch convoy's progress.
People thronged the streets of Falmouth, sometimes crowding around the torch and its bearer.
Officers from the Torch Security Team, co-ordinated by the Metropolitan Police, who are accompanying the runners, have been moving some spectators out of the way.
Earlier, they tackled a man who they thought was trying to reach the torchbearer.
Officers grabbed him and pushed him into a hedge.
A spokesman said: "We can confirm officers from the Met's Olympic Torch Team took action to prevent a man from gaining access to the torchbearer close to Land's End."
'Amazing atmosphere'
Devon and Cornwall police said a 3,500-strong crowd had gathered at Land's End in the early morning.
By lunchtime, the first torch used on day one of the relay had appeared for sale on eBay, complete with uniform.
The Olympic flame arrived on Friday evening on board a special BA flight from Athens to Royal Naval Air Station Culdrose, Cornwall.
Police have begun an investigation into reports that a red light was shone at the plane as it came in to land.
The flame spent the night on the base before being flown to Land's End by a Royal Navy search and rescue helicopter, which had one gold-painted rotor blade, early on Saturday morning for the start of the relay.
It was carried to the world-famous signpost, where Ainslie's torch was lit shortly after 07:00 BST.
The sailor, who on Friday won a record sixth Finn world title, said: "It's something I'll never forget. It was an amazing atmosphere.
"But it's back to reality tomorrow and training for the Olympics."
Sarah Blight holds the torch aloft near St Michael's Mount
He ran a short distance before passing the flame and wishing good luck to the second torchbearer, 18-year-old Cornish surfer Tassy Swallow.
Ms Swallow from St Ives said she wanted to take it slowly to take it all in but she got "a bit excited and a little crazy and ran too fast".
She has represented Britain four times as member of the junior British surf team and dreams of becoming the first woman to represent surfing and snowboarding for her country.
She told the BBC: "There's a good spot near here if there are some waves. I'll maybe check that out on the way home."
Penzance relay runner Marie Wilson said after handing over the torch: "It's so amazing. I was crying so much I was worried I was going to put out the flame."
A family at Sennen turned out to watch the torch go by
But some spectators in Penzance were left disappointed after the Olympic torch relay did not appear on their street - including disabled people from St Teresa's Home in Long Rock, who got up early to line the pavement in their wheelchairs.
Cornwall Council blamed a "confusion of maps" for the mix up which saw the torch convoy pass by the road.
A council spokeswoman said: "It was entirely our fault and we have apologised to the home. We were devastated as we spent a lot of time finding a place for them."
Eden Project balloon
During the day, the torch will be borne by more than 100 people including Olympians Duncan Goodhew and rower Michael Lapage who won silver at the 1948 Games.
TV presenter Ben Fogle and six of the Military Wives choir, who had a Christmas number one single, will also carry the torch in the county.
Mr Fogle carried the flame in a helium-filled balloon in the Eden Project rainforest biome.
Hip Hop artist Labrinth will headline the first evening celebration on Plymouth Hoe which starts at 19:00 BST.
The free two-hour stage show will also feature Britain's Got Talent finalists Twist & Pulse.
Later, the torch will visit BBC Radio 1's dance party at Paignton, Torbay.
David Beckham lit a 2012 torch and a cauldron from the flame at RNAS Culdrose
On Friday evening there was a short ceremony at Culdrose as the Olympic flame landed from Athens on board flight BA2012.
The Princess Royal carried it off the specially-painted plane in a lantern and footballer David Beckham then lit a cauldron.
Deputy Prime Minister Nick Clegg was at the Royal Naval Air Station to formally welcome the flame with 500 members of the public, armed forces and dignitaries.
He said: "Eight thousand people will pass it from hand-to-hand, a human chain that reaches the length and breadth of Britain.
"With every step, the excitement will build. Ten weeks from now, the world will watch as the flame arrives at the Olympic Stadium, bringing with it the hopes of a nation."
Chairman of Olympic organisers Locog, Lord Coe, who went to Greece for the lighting and handover of the flame, said this next stage in the build-up to the Games was a "magical moment for any host country".
Sun's rays
The torch visits the four nations of the UK before being taken to the Olympic Stadium in Stratford on 27 July for the opening ceremony of the Games.
It will travel through 1,019 cities, towns and villages, on foot or in convoy, and drop in at UK landmarks like the Giant's Causeway and Stonehenge.
It will be carried by 8,000 bearers, or taken in a convoy and will also be transported by boat, bicycle, tram and train.
The flame, meant to represent purity, was kindled from the rays of the sun using a parabolic mirror in a ceremony on 10 May at Olympia, the home of the ancient Olympic Games.
It was taken on a 1,800 mile-long relay around Greece before being handed over to the Princess Royal on Thursday evening at the Panathenaic stadium, Athens.
A flame first burned for a modern summer Olympic Games at Amsterdam 1928, but it was not until Berlin 1936 that a full torch relay was staged.
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