Determined to put on a show of "solidarity" for Boston, London Marathon organizers will stage the race on Sunday.
The British capital has long been a top target for terrorists, and these concerns have only intensified after harrowing scenes from the Boston Marathon on Monday, where bombs killed three people and injured more than 170.
London organizers confirmed on Tuesday that the London version will go ahead as planned after holding high-level talks with police and authorities. Earlier, British sports minister Hugh Robertson said the nation "won't be cowered by this sort of behavior."
"The best way for us to react is to push ahead with the marathon on Sunday, to get people on the streets and to celebrate it as we always do in London," said Robertson, who hailed Britain's security forces as "the best anywhere in the world."
"These are balance of judgments but we are absolutely confident here that we can keep the event safe and secure. I think this is one of those incidents where the best way to show solidarity with Boston is to continue and send a very clear message to those responsible."
The London Marathon will be watched by an estimated 500,000 spectators as 37,500 runners weave their way past and along some of the city's most recognizable landmarks. Prince Harry, the patron of the London Marathon Charitable Trust, will attend the race and make the presentations to the winners.
"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," Nick Bitel, chief executive of the London Marathon, said in a statement.
"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."
Mo Farah, who won the long-distance double at the London Olympics, will be running a half-marathon. The three medalists from the men's marathon at the London Games are also among the entries.
There have been no high-profile withdrawals so far and the buildup to the marathon remains unchanged for competitors.
"There's rather intense activity going on to ensure the race is safe and is carried off with the usual aplomb," said Ed Warner, chairman of UK Athletics. "The decision at the moment is to go ahead with the race and I'm sure it is the right decision. They will cope very well with the increased demands placed on them."
The race Sunday in London is one of six in the world marathon series along with Tokyo, Boston, Berlin, Chicago and New York.
Belgrade is among other cities staging marathons this weekend, with organizer Dejan Nikolic expressing confidence that the race will be safe and a "beautiful running festival."
"We will do our best so that this year the security level is even higher," Nikolic said.
In 2005, suicide attacks on the public transportation system in the British capital killed 52 people. London and mainland Britain also face a moderate threat from Northern Ireland-related terrorism, according to the government, and a massive security operation was put in place last summer to protect the successful London Olympics.
The Boat Race between English universities Oxford and Cambridge went ahead this year amid tightened security after a protester jumped into the River Thames last year and narrowly avoided being hit by the oars of the two crews. Royal Marines were stationed along the length of the 4 1/4-mile course.
Police already were preparing a major security operation in London for the ceremonial funeral on Wednesday for former Prime Minister Margaret Thatcher, an event at St. Paul's Cathedral that will be attended by Queen Elizabeth II and dignitaries. The plans call for a procession through the streets of London, with Thatcher's flag-draped coffin to be carried on a horse-drawn carriage.
The funeral security plans are expected to be reviewed in light of the bombings at the Boston Marathon.
"I would stress that at this stage, there's been no announcement that this is terrorism-related," Metropolitan Police commander Christine Jones said. "We are working with London Marathon to make sure we have all the tactics in place that we need."