Around 36,000 runners were taking part in the London race, the first in the World Marathon Majors series since two explosions near the finish line of the Boston Marathon on Monday killed three people and wounded 176.
London's Metropolitan Police Service almost doubled the number of officers sent out to secure the event, saying this was to reassure the public and not a response to a specific threat.
Before the start of the men's elite and mass races, official commentator Geoff Wightman led the crowd in a tribute to Boston.
"This week the world marathon family was shocked and saddened by the events at the Boston Marathon," he said over loudspeakers.
"In a few moments a whistle will sound and 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 packed ranks of competitors bowed their heads and stood silently for 30 seconds, then clapped and cheered when a second whistle marked the end of the tribute.
Seconds later, the world's elite runners led off the race. Behind them came thousands of competitors chasing personal goals or raising money for charity, many running in fancy dress including challenging two-person camel and horse costumes.
"It is a bit different this year because everybody's aware of what happened in Boston ... They're not going to stop us running," said Steve Williamson, a three-times competitor in the London Marathon now taking part as a marshal.
"It was incredible, the amount of support, people coming out from everywhere, just cheering the whole way. Unbelievable," said a breathless Mo Farah, Britain's 5,000 and 10,000-metre Olympic champion, after running the first half of the course.
Farah ran half of the route to prepare for competing next year.
"Wishing all involved in London Marathon a great day out, good luck if you're raising money and Boston Marathon our thoughts are with you today," said Boris Johnson, the Mayor of London, on Twitter.
After an unusually long and harsh winter, the weather came through for the marathon which began under bright sunshine and a cloudless sky, a bonus for the competitors and for the hundreds of thousands of spectators expected to cheer them on.
The 26-mile course started in leafy Greenwich, crossed Tower Bridge, snaked through the Canary Wharf business district before going through the heart of London, past Big Ben to Buckingham Palace.
Ethiopian Tsegaye Kebede won the men's race while Kenya's Emmanuel Mutai was second, 29 seconds behind, with another Ethiopian Ayele Abshero third almost a minute off the pace.
Kenya's Olympic silver medallist Priscah Jeptoo won the women's title in the quickest time this year, while world champion Edna Kiplagat was runner-up for the second successive year and Japan's Yukiko Akaba took third.
Police with sniffer dogs were out in force and bins had been removed from the length of the course as part of enhanced security.
"The enhancement to policing, which will see several hundred additional officers on the streets, is intended to provide visible reassurance to the participants and spectators alike," the Metropolitan Police said on its website.
There was a 40 percent increase in officers on the street compared with what was planned before the Boston bombings.
Before the race, competitors picked up their runner numbers and kit bags at the ExCel convention centre, where they were also provided with black ribbons.
At a message board inside the Excel, some runners had posted messages of solidarity such as "We'll be thinking of those in Boston" and "Praying for Boston".
The organisers are donating £2 per finisher to The One Fund Boston, set up to raise money for the victims. They estimate around 35,500 people will cross the line, meaning they are likely to raise at least £70,000.
In the German city of Hamburg, which was staging its own marathon on Sunday, runners also wore ribbons and held a minute of silence for victims of the Boston bombings.