Exhausted participants flood the Mall in central London after finishing the London Marathon on April 21, 2013. Lauren Said-Moorhouse/CNN

American Greg Takacs, 36, ran in the Boston Marathon last week and proudly wore his Boston race shirt adorned with black ribbons. Describing the 30-second silence at the start of the race, he told CNN: "It was so silent and very moving ... For that many people to be that quiet -- it was incredible. Wow." Lauren Said-Moorhouse/CNN

Thousands of London Marathon runners paid tribute to Boston this morning wearing black ribbons and observing a 30-second silence on the start line. Here one athlete customized her shirt with the words 'For Boston' on the back. Lauren Said-Moorhouse/CNN

Priscah Jeptoo of Kenya runs down the Mall towards finish line and victory in the Womens Elite section during the Virgin London Marathon 2013 on April 21, 2013 in London, England. Jackson/Getty Images

Thousands of runners took part in today's marathon through London, many wearing black ribbons as a symbol of respect to the Boston victims following the tragic events on Monday. Lauren Said-Moorhouse/CNN

Bill Higgins, 62 from California is a seasoned marathon runner having competed in 84 races. He told CNN: "I've run Boston three times and (the Boston terror attack) really hit home. It's a thrill and an honor to be here." Lauren Said-Moorhouse/CNN

Virgin Group founder Richard Branson was at the finish line to greet runners as they finished the 26.2-mile race. Lauren Said-Moorhouse/CNN

Many American runners proudly dressed in a stars and stripes in a show of solidarity following the Boston terror attacks Monday. Kevin Taverner/CNN

British runner Julia Biss saw this tribute to Boston being passed around on Facebook which urged runners to print off the sign and wear it during the London Marathon on April 21, 2013. Lauren Said-Moorhouse/CNN

In addition to wearing black ribbons, many athletes chose to hold their hand over their heart as they crossed the finish line. Lauren Said-Moorhouse/CNN

John O'Flynn (L) and John Gately (R) came from Cork, Ireland to participate in the London Marathon on April 21, 2013. Both have family in Boston but were not planning on competing, due to injury, until last week's tragic event changed their mind. O'Flynn said they wanted to compete to show solidarity to the people of Boston. Lauren Said-Moorhouse/CNN

Several marathon runners also competed in the Boston Marathon Monday and wore their shirts in London. Lauren Said-Moorhouse/CNN

First time London Marathon runners Barry McCaan, 27, Aidan Savage, 31, Martin Conway, 30 ahead of the race. McCaan told CNN: "Everyone has been so generous. It totally makes you more determined to show them [the bombers] that terrorism doesn't work." Lauren Said-Moorhouse/CNN

Fancy dress runners compete on April 21 in the London Marathon which started in East London before traveling into the city, along the River Thames and finishing near Buckingham Palace. Kevin Taverner/CNN

Runners stand in a silent tribute to the victims of the Boston Marathon bombings before the start of the London Marathon from Greenwich Park on April 21, 2013 in London, England. Peter Macdiarmid/Getty Images

Athletes bow their heads at the start in Blackheath Common while observing the 30-second silence to remember those killed and injured by the blasts near the finish of the Boston Marathon on Monday. Lauren Said-Moorhouse/CNN

Start of the Elite Men in the Virgin London Marathon 2013 on Blackheath Common on April 21, 2013 in London, England. Steve Bardens/Getty Images

A handler works with an explosive-detecting dog on The Mall in central London on April 20, 2013, on the eve of the London Marathon. A beefed-up police presence for the London Marathon on April 21 will remain in place despite the death and capture of the Boston Marathon bombing suspects, Scotland Yard said. LEON NEAL/AFP/Getty Images

A handler works with an explosive-detecting dog on The Mall in central London on April 20, 2013, on the eve of the London Marathon. London's Metropolitan Police is putting in place hundreds more officers along the route compared with last year and search dogs to reinforce security, in a bid to reassure the 36,000 runners and tens of thousands of spectators. AFP PHOTO / LEON NEALLEON NEAL/AFP/Getty Images

British Olympic double gold medalist Mo Farah poses for photographers in central London on April 18, 2013 during a photo call ahead of the London marathon. The London Marathon will go ahead as planned on April 21, 2013 after security arrangements were reviewed in the wake of the bombings that caused carnage at the Boston Marathon, organizers and police said. CARL COURT/AFP/Getty Images

Tiki Gelana, Edna Kiplagat, Priscah Jeptoo, Renee Baillie and Yoko Shibui attends the photocall for International Women photocall ahead of The the London Marathon at The Tower Hotel on April 18, 2013 in London, England. Ben A. Pruchnie/Getty Images