Runners and spectators observe a 30-second silence at Blackheath.
Thousands of runners have taken part in the London Marathon 2013, many with black ribbons on their vests in honour of the victims of the Boston bombings.
A record 700,000 spectators lined the streets as the 36,000 runners set off from Blackheath.
Hundreds of extra police officers and sniffer dogs were drafted in as reassurance in London.
Three people died and more than 170 were injured by Monday’s blasts, near the finish line of the Boston Marathon.
Just before the start of the main race, event commentator Geoff Wightman introduced the half-minute silence.
“Marathon running is a global sport,” he said.
“It unites runners and supporters on every continent in pursuit of a common challenge and in the spirit of friendship and fellowship.”
He said the marathon family had been “shocked and saddened” by the events in Boston and the silence was to show “our respect and support for the victims”.