30 seconds of silence marked the start of the London Marathon on Sunday to commemorate those killed in the recent Boston bombings.
Thousands lined the streets to see the athletes set off from Blackheath, many wearing black ribbons on their vests.
Hundreds of extra police officers – 40% more than last year – have been drafted in to reassure runners and spectators.
About half a million people will watch the race, which starts at Blackheath in south-east London and finishes near Buckingham Palace, passing some of the city’s most recognisable landmarks, including Tower Bridge and Big Ben.
Chef Michel Roux, who appears as a judge on the BBC’s MasterChef, is one a number of celebrities taking part.
He said his response to the Boston bombings on Monday had been one of “shock and horror”.
“I immediately thought of the families of all those who perished. It was just atrocious. But my reaction after that was London. We’re going to do it and we’re going to do it for them.”
The Metropolitan Police said that it had reviewed security plans after the Boston Marathon attacks and that the extra officers would be used for “for reassurance patrols”.
Ch Supt Julia Pendry, who has headed marathon security for five years, has said this year’s police operation is “about making sure that people who come to London on Sunday feel safe when they are in the city”.
She said more search dogs would be deployed and urged marathon spectators to keep their belongings with them to avoid sparking security alerts.
Extra security has been in put in place around the bags of the runners, with 34 lorries taking the rucksacks of the thousands of runners from the start in Blackheath to the finish on the Mall near Buckingham Palace.
Virgin London Marathon has pledged to donate £2 for every runner that finishes Sunday’s event to The One Fund Boston set up to raise money for victims of the explosions. The race organisers expect the total donation to be at least £70,000.