Security for Sunday's London Marathon will be reviewed after two fatal explosions hit the Boston Marathon, but the event will go ahead, officials say.
At least three people were killed and more than 100 injured by the blasts near the finish of Monday's US event.
About 375 Britons were listed as taking part, but there has been no confirmation of any British casualties.
Sports Minister Hugh Robertson said he was "absolutely confident" the London Marathon could be kept safe.
London had "enormous experience" of delivering major events and the UK had some of the best security professionals in the world, he told the BBC, adding: "This is one of those instances where the best way to show solidarity with Boston is to continue."
Home Secretary Theresa May has been briefed on the bombings by Scotland Yard Assistant Commissioner Cressida Dick and MI5 chief Sir Jonathan Evans.
At this stage, there are no plans for a meeting of the government's emergency committee Cobra, but high-level discussions were already planned and will go ahead because of Baroness Thatcher's funeral on Wednesday.
The Metropolitan Police said security for the funeral in central London would not be affected by the Boston explosions and that it was a very different event.
London Marathon chief executive Nick Bitel said in a statement: "We are deeply saddened and shocked by the news from Boston."
He later told the BBC that he "fully expected" the London Marathon, which first took place in 1981 and was completed by more than 37,000 people last year, to go ahead.
He said security plans "take account of many contingencies, including this type of threat and incident, but one can't be complacent and when it has happened, you need to then review those plans you have in place to see what else may be necessary."
St James's Palace said Prince Harry would still attend the marathon to make the presentations to the winners.
The London Marathon route, which is lined by hundreds of thousands of spectators every year, starts in Blackheath and finishes near Buckingham Palace, passing some of the capital's most recognisable landmarks including Tower Bridge, Canary Wharf and Big Ben.
Of the deadly Boston blasts, UK Prime Minister David Cameron said: "The scenes from Boston are shocking and horrific - my thoughts are with all those who have been affected."
London Mayor Boris Johnson described the bombings as "shocking, cowardly and horrific".
He has spoken to Met Police Commissioner Sir Bernard Hogan-Howe about the possibility of extra security for the London Marathon.
"We do have robust security measures in place, but given events in Boston it's only prudent for the police and the organisers of Sunday's race to re-examine those security arrangements," he said.
'Runners offered blood'
According to the Boston Marathon website, 374 of the more than 25,000 registered entrants to one of the US's biggest sporting events were British.
UK Athletics confirmed that none of its elite runners had taken part.
Keith Luxom, part of a group of 24 British runners, had already finished when the bombs went off. He told BBC Breakfast the jubilant mood changed to one of "shock, horror and total disbelief that people would target this great institution".
"Lots of runners went to the hospitals and offered blood," he said. "People were sharing mobile phones so people without one could phone home."
Mr Luxom, from Essex, says he will be running the London Marathon this weekend and he will be wearing his Boston finishing shirt.
Sports Tours International, which organised a trip for 51 people - 40 runners and 11 non-runners - said all were accounted for.
David Mouncey, from Bristol, who finished the marathon 15 minutes before the explosions went off, told the BBC: "I hadn't planned to do Boston next year, but I would very much like to go back to support the Boston people and show that [I'm] not cowed by what's happened."
Another runner, Owain Griffiths, from Cyncoed in Cardiff, said "a day of enjoyment" had descended into "unthinkable horror".
Marathon world record holder Paula Radcliffe posted on Twitter: "Situation looks awful, thoughts with everyone. There are some very sick people out there, who would do something like this?"
Police say they have also launched a review of security for the Greater Manchester Marathon, due to take place on Sunday 28 April.