The London Marathon will go ahead this weekend despite the bombings that caused carnage at the Boston Marathon, but security has been reviewed, organisers and police said on Tuesday.
Three people were killed and more than 170 wounded, some with horrific injuries, in twin explosions as runners approached the finish line in Boston on Monday, in the worst attack in the United States since the September 11, 2001 atrocities.
The FBI vowed to go "to the ends of the Earth" to find those responsible.
The organisers of the London race said after talks with police and city officials that the race will take place as planned on Sunday.
London Marathon chief executive Nick Bitel said: "The support we have been offered by our stakeholders and the wider running community has been outstanding. We have the full support of the Metropolitan Police, the mayor's office and other authorities.
"We want to reassure our runners, spectators, volunteers and everyone connected with the event, that we are doing everything to ensure their safety and that the Virgin London Marathon 2013 is an outstanding success."
Commander Christine Jones from the Metropolitan Police said the force was working with the organisers to make sure "we have all the tactics in place that we need".
The head of the force, Bernard Hogan-Howe, said it would be "professionally irresponsible" if it did not re-assess its security arrangements for the London Marathon in light of the attacks in Boston.
It was an additional security headache for the London police, who are already on alert for Wednesday's funeral of former British prime minister Margaret Thatcher.
The London race, which is expected to attract 36,000 runners and half a million spectators, has long been considered a potential target for terrorists.
Centred around the River Thames, the course winds past several of London's most iconic landmarks, including Tower Bridge, the Houses of Parliament and Queen Elizabeth II's Buckingham Palace residence.
It also passes through Canary Wharf, one of the world's biggest business districts.
Bitel admitted earlier that the nature of the event made it difficult to secure the entire 26.2-mile (42.2-kilometre) route.
"When you have an event of any nature -- a marathon, parade -- it's only as safe as the city itself. If it's not held in a stadium, you can't do a lockdown like you may do in a building," he said.
Organisers must also consider security arrangements for third-in-line to the throne Prince Harry, after St James' Palace confirmed he still planned to attend the event to present medals to the winners.
Sports Minister Hugh Robertson, who oversaw security arrangements during last year's London Olympics, said the city had substantial experience of dealing with the risk of terror attacks.
"There has been a terrorism threat every day of my adult life here in London -- first through (Irish) Republicanism, then through international terrorism. There are major events in London -- if not quite on a daily basis, certainly on a weekly basis," he told BBC radio.
He added: "These are balance of judgements, but we are absolutely confident here that we can keep the event safe and secure."
British government officials including Home Secretary Theresa May met representatives from the security services to review the arrangements.
Mayor of London Boris Johnson said the bombings in Boston were "shocking, cowardly and horrific" and Londoners' thoughts were with the victims.
"We do have robust security measures in place for Sunday's London Marathon 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.
The London authorities will draw on their experience of hosting last year's Olympics, which drew over half a million visitors to the city but passed off without any major security incidents.
As well as thousands of amateur runners, the London Marathon is set to attract a world-class field of elite athletes, including all three medal-winners from the men's race at the Olympics.
British star Mo Farah, who claimed the 5,000m and 10,000m double at the London Games, is also due to complete half the race, as he begins preparations to compete over the full distance.