Money raised by London Marathon runners since its conception broke the STG1 billion ($A1.8 billion) mark as tens of thousands of people took part in the 39th event.

Elite runners were joined by celebrities and amateur fundraisers as race officials accepted the highest ever number of entrants.

Tennis star Andy Murray pushed the starting button to launch the Elite Men, British Athletics and England Athletics Marathon Championships, and Mass Race at 10.10am on Sunday.

British Olympic champion Mo Farah finished fifth, with a time of 02:05:39, behind Kenyan winner and marathon world record holder Eliud Kipchoge, who crossed the line in 02:02:37.

Earlier in the morning, the Elite Wheelchair race and World Para Athletics Marathon Championships Ambulant Athletes set off followed by the Elite Women at 9.25am.

The Duke of Sussex also made a surprise appearance to hand out medals to marathon and wheelchair winners and meet volunteers. Among them was Australian Michael Roeger, who won the T45/46 category.

Prince Harry had always planned to attend but his visit was uncertain as he awaits the birth of his first child.

Royal fans used the prince’s appearance to speculate his wife Meghan, who is at their home Frogmore Cottage on the Windsor Castle estate, would not give birth imminently.

The London Marathon Charity of the Year 2019 was Dementia Revolution, a partnership between the Alzheimer’s Society and Alzheimer’s Research UK.

Their STG3.5 million fundraising target was bolstered by a team running for actress Barbara Windsor, 81, who was diagnosed with Alzheimer’s disease in 2014.

Her husband, Scott Mitchell, and a team of EastEnders cast members including Adam Woodyatt, who plays Ian Beale, had raised STG100,000 before the race started.

French tennis ace and Wimbledon winner Amelie Mauresmo, who coached Andy Murray, said: “Every marathon I have run so far, at the finish line, I’ve said I will never do another marathon and I certainly feel like that now.”

Mauresmo finished in 03:22:45.

The marathon is the United Kingdom’s largest annual fundraiser and a ballot of 444,168 applicants was narrowed down to 56,398 who were given a place.

With AAP.