World-class athletes and fun-runners alike are to stand in silence for 30 seconds at 10:00am (0900 GMT) 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 percent -- no numbers have been given -- following the Boston attacks on Monday.
And they have decided to stick with the new plan despite the death and capture of the two suspects.
Scotland Yard hopes 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 ($3, 2.30 euros) for every finisher will be donated to a fund for victims of the Boston attacks.
The silence will be observed at all three marathon starts in Greenwich Park and Blackheath.
"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 a young, ethnic Chechen man suspected of staging the Boston bombings, after a desperate 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, including several victims with severed limbs.
On the road, much of the focus will be on Britain's 10,000 and 5,000 metres men's Olympic champion Mo Farah, even though he will only be running half the distance.
Competitors going the full 26 miles and 385 yards, or 42.2 kilometres, include six of the 10 fastest marathon runners ever, with Kenya's Wilson Kipsang wearing bib number one.
He won last year's men's race, missing out on Emmanuel Mutai's course record by four seconds, while compatriot Mary Keitany won the women's in an African record marathon time.
Kipsang's chief rivals are likely to be his compatriots Patrick Makau, Geoffrey Mutai, Emmanuel Mutai and three-time London winner Martin Lel, along with London 2012 Olympics champion Stephen Kiprotich.
Ethiopians Ayele Abshero, Tsegaye Kebede and Feyisa Lilesa are also likely to be in the leading pack.
In the women's field, London Olympics champion Ethiopia's Tiki Gelana leads a field of four athletes who have run under two hours and 20 minutes, along with Yoko Shibui of Japan and Kenya's Florence and Edna Kiplagat.
This year's winners will receive $55,000 each from the total $313,000 pot.
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.
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