London Marathon organisers were on Tuesday grappling with the conundrum of securing the race's 26.2-mile (42.2-kilometre) route after the fatal bomb blasts that devastated the Boston Marathon.
Three people were killed and more than 100 injured in twin explosions as runners made their way towards the finish line in Boston on Monday, sparking fears of a coordinated terror attack.
The organisers of the London race said it would go ahead despite the carnage in Boston.
But with those responsible for the blasts still at large and their motives unknown, British government officials including Home Secretary Theresa May met representatives from the security services to review plans for Sunday's race through the streets of the British capital.
"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," said Mayor of London Boris Johnson.
Security has been a major concern at high-profile sporting events in Britain ever since the September 11, 2011 terror attacks on the United States and particularly in light of the July 2005 bombings on London's transport network, which killed 52.
The London Marathon, which is expected to attract 36,000 runners and 500,000 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.
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, where he will present medals to the winners.
The British police are already on high alert for the funeral procession on Wednesday of former prime minister Margaret Thatcher, when protests are expected.
London Marathon chief executive Nick Bitel acknowledged the difficulties posed by the nature of the event, saying: "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 the race had "fairly detailed contingency plans", but admitted: "When something of this nature does happen, you obviously want to review them and see if changes need to be made."
The London authorities will draw on their experience of hosting last year's Olympic Games, which drew over half a million visitors to the city but passed off without any major security incidents.
More than £1 billion ($1.5 billion, 1.2 billion euros) is estimated to have been spent on securing London during the Games.
British Sports Minister Hugo Robertson, who oversaw security arrangements during the 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."
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 last year's Olympics.
British Olympic 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.
Former Olympic athlete Steve Cram urged people planning to compete not to be deterred by the attacks in Boston.
"I think people will be concerned, people who would either be coming to spectate or take part, but I know the team at London very, very well and London is renowned as being one of the best-organised events in the world," Cram told the BBC.
Counter-terrorism expert Richard Barrett, senior director at the Qatar International Academy for Security Studies (QIASS), said: "You've had all too many experiences with the actual attacks, so certainly security will be ramped up from the already extraordinarily high levels originally planned for these events."