An estimated 37,500 fun runners, amateur athletes, celebrities and elite sportsmen and women are taking part in the Virgin London Marathon.
The elite women runners began their race at 0900 BST, the elite wheelchair race started at 0920 BST and the elite men set off at 0945 BST.
Among the runners, more than 100 people will be attempting to break world records, said Guinness World Records.
They include the record for the fastest marathon on stilts.
Sunday’s race, which remains the world’s largest annual fund-raising event, is the last chance for elite British runners to claim a place in this summer’s Olympics.
Only one man, Scott Overall, has so far qualified for Team GB and he is the official London Marathon pace maker.
Rugby league stars Steve Prescott and Paul Sculthorpe are hoping to complete a grueling eight day challenge with the marathon.
Prescott, 38, was diagnosed with a rare form of stomach cancer in 2006 and is aiming to raise £16,000 for his charitable foundation.
The former St Helens players have already completed the Paris marathon, cycled 184 miles through France, and kayaked across the English Channel.
The pair then pedaled another 82 miles before setting off on the London race.
Prescott said: “We’re pretty tired but it’s been an incredible journey.”
Charley Phillips, 27, and George Phillips, 22, are aiming to break the current record for the fastest time on stilts – eight hours and 25 minutes.
The brother and sister from Hammersmith, west London, learnt circus skills as children and are hoping to raise £5,000 for Macmillan Cancer Support in memory of their mother, who died of lung cancer.
Mr Phillips said the pair had suffered from blisters down their legs from the stilts attachments and that manhole covers, cobbles and water bottles would be problematic, particularly if it rained.
Before the race he said: “We went to Kensington Gardens and Hyde Park to train and people asked us if it was some kind of crazy exercising regime.
“When we said it was for the marathon they said we must be insane.”
Sasha Kenney, 34, hopes to raise more than £2,000 for the NSPCC as she attempts to break the six-hour mark for the quickest hula hooping runner.
“Someone said to me it couldn’t be done,” said Ms Kenney, a fitness instructor from Wrexham, north Wales.
“I see a lot of that in my profession so I want to prove them wrong.”
BBC newsreader Sophie Raworth, 43, said she hoped to “stay upright” after collapsing during last year’s event.
She crashed into a barrier on the side of the road and blacked out when she reached the 23-mile mark in 2011. She is running for St John Ambulance.
Model Nell McAndrew is taking part in her sixth London Marathon.
Her times have been so quick that she has qualified to compete with the elite athletes and is trying to break the three-hour mark for the first time.
And Shadow Chancellor Ed Balls is running his first London marathon, on behalf of two organisations, Action for Stammering Children and Whizzkidz.
He said that despite all his experience in politics and the public eye it was a daunting task.
The 26.2 mile long route ends in The Mall.