Security for the London Marathon and Baroness Thatcher's funeral will be under scrutiny in the aftermath of the Boston Marathon bombs that have killed three people and injured as many as 140 others.
The Metropolitan Police said they would review the policing of the race on Sunday, while former Home Secretary John Reid said the incident will also have ramifications for Lady Thatcher's funeral on Wednesday.
Marathon organisers and police have been liaising ahead of the event in light of the incident in Massachusetts on Tuesday afternoon.
The route passes some of the nation's most recognisable landmarks and organisers said they will assess safety arrangements for the event following the tragic scenes that marred the finish line in Boston.
Chief Superintendent Julia Pendry, the event commander for the marathon, said they would review security for the event, which draws spectators from all over the world.
She said: "A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon."
The London race's chief executive, Nick Bitel, said it was "a very sad day for athletics and for our friends in marathon running".
Mr Bitel said: "Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news."
Meanwhile, Lord Reid has said experts will be keen to review security arrangements for Lady Thatcher's funeral, which is also expected to pass major London landmarks.
He said: "Everyone will have been appalled at the terrible events in Boston. While the culprits and motivation behind the US terror attacks are still unclear, this will obviously entail a review of security arrangements for both Lady Thatcher's funeral and the London Marathon.
"In light of the awful attacks in Boston they will require extra vigilance from everyone involved and it is also to be hoped that everyone recognises the added responsibility of co-operating with the police and the authorities at both events."
David Lowe, a specialist in security at sports events, said security at the funeral of Lady Thatcher and at the London Marathon would be as "tight" as possible.
"I think you are going to find a lot of surveillance on the crowds, it is the classic of not those watching the race - those who may be looking around, acting slightly different, slightly more suspicious," he told BBC Breakfast.
"There could be people looking elsewhere for totally different reasons - I can imagine that the security will be stepped up.
"Of course we have the funeral of Margaret Thatcher tomorrow, you will have people protesting who feel vehemently about her premiership.
"Again, there will be high security for that because there will be people lining the streets to pay tribute to Margaret Thatcher.
"We have the two events where we have got to make sure, certainly the security services and the police, have got to make sure it is as tight as possible."
The London Marathon is the world's largest marathon and attracts half a million spectators each year.
The flat and fast course, starting in Blackheath and finishing in front of Buckingham Palace, takes in some of the capital's top tourist sights including Tower Bridge, and Canary Wharf - one of the largest single business developments in the world.
Runners also pass Big Ben and Parliament Square.
The event first ran in 1981, and Sunday's will be the 33rd race.