Illusionist David Blaine has begun his latest stunt, encased in a plastic box over the River Thames.
Blaine will only be given water
Blaine entered the box at around 2100 BST on Friday night.
He will spend 44 days suspended beside the London river.
During that time he will apparently go without food and will only have access to water fed through a tube.
Sky One showed live coverage of Blaine's entry into the box on Friday evening and will show highlights for the following 44 days, with Channel 4 showing the footage three days later.
Hundreds of spectators gathered to watch him enter the box.
He told them: "The feeling of wonder is amazing ... to push ourselves further. I'm going to push myself as far as I can."
His exit is due to be shown live on Sky One on 19 October and on Channel 4 the following day.
Click here to see how Blaine will carry out his stunt
The Perspex box Blaine has taken up residence in measures just 7ft by 7ft by 3 ft (2.1m by 2.1m by 0.9m) and is be suspended by a crane near to Tower Bridge.
Bookmakers Ladbrokes is refusing to take bets on the challenge because it has no way of verifying whether Blaine will genuinely survive without food.
"In our eyes Blaine is odds-on to complete his feat but we need convincing that the stunt is a true test of endurance," said Ladbrokes spokesman Warren Lush.
"For example, how pure is the water he is drinking? A bookie won't get involved unless we are convinced that there is a wholly independent steward or referee.
Last year he leapt from an 80ft pole in New York
"We would love to take bets on Blaine but bookies won't mess with an illusionist otherwise, by magic, we will find ourselves shelling out thousands of pounds."
But William Hill is offering odds of 2-9 that Blaine will successfully complete the challenge, and 3-1 that he will come out early.
The Guinness Book of Records has refused to recognise the stunt.
It says it will not condone a "fasting record."
The Mayor of London, Ken Livingstone, has said he is against the stunt.
He says David Blaine shouldn't put his life in danger.
Other critics say Blaine's attention-seeking behaviour has no political aims and is therefore disrespectful to the many people around the world who really are starving or isolated.
The stunt is the latest in a series of bizarre challenges the US magician has set himself.
In 2000, Blaine spent 61 hours, 40 minutes, and 15 seconds
encased in a six tonne block of ice in New York's Times Square.
And, in 1999 he was buried six feet down under a glass tank filled with water for a week, being fed just four tablespoons of water a day.
For his latest challenge he is attempting to stay without food for more than six weeks, with no communication or distractions.
The box contains trickling water and a tube for him to urinate - by which doctors will assess his health.
The BBC's Rachel Ellison says starvation will weaken the magician's muscles and could cause organ failure.
Prior to entering the box, Blaine said he weighed 205 lb (93 kg) having begun an added glucose diet.
He is expected to lose about 45 lb (20 kg) during more than six weeks without food.