The London Marathon takes place amid a heightened police presence following the Boston Marathon bomb attacks, with the 36,000 runners observing a 30-second silence to remember victims.
World-class athletes and fun-runners alike are to stand in silence for 30 seconds at before the starter's flag drops on the elite men's and mass races.
Scotland Yard police headquarters decided to boost the number of officers by 40 per cent - no numbers have been given - following the Boston attacks on Monday. They hope the reinforced security along the route will reassure the 36,000 runners and tens of thousands of spectators.
Besides the extra officers, police search dogs will also be deployed.
Prince Harry, third in line to the throne, is set to present the medals. His office said they have not been notified of any changes to the plan following the Boston bombings.
Runners have been encouraged to wear black ribbons and organisers have announced that 2 pounds for every finisher will be donated to a fund for victims of the Boston attacks.
With more than 36,000 runners taking part and most expected to finish, they expect to arise around 70,000 pounds.
"We want to show our support for our friends and colleagues in Boston at this difficult time for the global running community," said race director Hugh Brasher.
"We are determined to deliver an amazing event that will focus on one of the core pillars of the London Marathon, which is to have fun and provide some happiness and a sense of achievement in a troubled world."
US police on Friday captured an ethnic Chechen man suspected of staging the Boston bombings with his brother, after a manhunt that virtually paralysed the city and its suburbs.
Police found Dzhokhar Tsarnaev, 19, hiding in a boat in a suburban backyard, wounded after a gun battle in which his brother was killed.
The bomb attacks left three dead and about 180 injured, many of the victims having lost limbs.
First held in 1981, the London Marathon course is relatively flat and fast.
The race starts in Greenwich Park and Blackheath, and passes the Thames Barrier and the Cutty Sark tea clipper before crossing the river at Tower Bridge.
The course heads out to the Docklands and back before a final few miles past the Tower of London, St Paul's Cathedral, the Houses of Parliament and Buckingham Palace.
Among the celebrities running are supermodel Sophie Anderton, Bucks Fizz singer Cheryl Baker, former Scotland football captain Christian Dailly, classical singer Katherine Jenkins, former Australia rugby captain Michael Lynagh, chef Michel Roux, former world superbike champion James Toseland, and 400 metres British record holder Iwan Thomas.
The oldest runners this year are 88-year-old Paul Freedman in the men's race and Iva Barr, 85, in the women's.