Security was tight for the London Marathon on Sunday as thousands of runners paid solemn tribute to the victims of the deadly bomb attacks at the Boston Marathon barely a week ago.
A sombre 30-second period of silence was observed ahead of the start of the elite men's and mass races in Greenwich Park on the south bank of the River Thames.
"We will join together in silence to remember our friends and colleagues, for whom a day of joy turned into a day of sadness," the runners were told over the public address system.
Olympic athletes and around 35,000 amateur competitors, many sporting black ribbons in memory of the Boston victims, bowed their heads in memory before streaming on to the streets of the British capital beneath bright blue skies.
One runner held aloft a banner bearing the words "For Boston".
Police numbers along the 26.2-mile (42.2-kilometer) course were increased by 40 percent after the twin bomb blasts during the Boston race on Monday, which left three people dead and around 180 injured.
As well as the black ribbon tribute to those affected by the Boston horror, a social media campaign has encouraged runners to place their hands on their hearts as they cross the finish line.
Organizers have also announced that £2 ($3) for every finisher will be donated to a fund for the Boston victims.
"Marathons are all about people coming together," Keith Luxon, an amateur runner who competed in the Boston Marathon and was also due to race in London, told the BBC.
"Part of that was ruined in Boston, and it's up to us to put some of that back."
The events in Boston prompted organizers of the London race to undertake a security review, but they said there was no known threat to the event.
"Obviously with the shocking pictures (from Boston), it galvanized us into having a look at our security measures," said London Marathon chief executive Nick Bitel.
"We've had an amazing response from the police, from the mayor (Boris Johnson), and from the other security agencies, but also from the wider community.
"We've had to make a few changes and put some new security measures in, but we've had such an amazing response from the runners and the public."
The marathon course snakes past famous London landmarks including Tower Bridge, St Paul's Cathedral and the Houses of Parliament before finishing in front of Queen Elizabeth II's Buckingham Palace residence.
Much of the attention is focused on British Olympic star Mo Farah, winner of the 5,000 meters and 10,000 meters at last year's London Games, even though he will only complete half the marathon distance.
Both the men's and women's races boasted star-studded fields, with defending champion Wilson Kipsang of Kenya starting as the favorite in the men's event.
Fellow Kenyans Emmanuel Mutai, the course record-holder, Geoffrey Mutai, world record-holder Patrick Makau and three-time winner Martin Lel will provide competition, along with London 2012 Olympic champion Stephen Kiprotich of Uganda.
In the women's race, London Olympic champion Tiki Gelana of Ethiopia led 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.
There was an early scare for Gelana when she fell after colliding with a men's wheelchair athlete at a drinks station, but she picked herself up and caught up with the leading pack.
Prince Harry, third in line to the British throne, is scheduled to present medals to the leading finishers in the men's, women's and wheelchair races.
His office said it had not been notified of any changes to the plan following the Boston bombings.
US police on Friday captured a 19-year-old ethnic Chechen man, Dzhokhar Tsarnaev, suspected of carrying out the Boston attacks with his brother, after a desperate manhunt that virtually paralyzed the city and its suburbs.
Police found Tsarnaev hiding in a boat in a suburban backyard, wounded after a gun battle earlier in the day in which his brother was killed.