Race organisers said the London Marathon would go ahead on Sunday despite the death of at least two people in explosions near the finish line of the Boston Marathon, but police said they would review security plans.
While it has yet to be confirmed that the Boston explosions on Monday were caused by a terror attack, the shocking scenes quickly prompted fears of a similar incident at the London race.
"We will not be cancelling, what we are doing, we are reviewing," London Marathon Chief Executive Nick Bitel told BBC Radio 5 Live.
"You look at what has occurred, if there are steps we can take to increase security and all sorts of measures one could deploy.
"We run through the city, 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 added.
Thousands of people, including leading international athletes, compete in the London Marathon every year and, with the race just six days away, there was earlier speculation the event could be cancelled on safety grounds.
Bitel said he was "deeply saddened and shocked by the news.
"Our immediate thoughts are with the people there and their families," he said in a statement. "It is a very sad day for athletics and for our friends and colleagues in marathon running.
"Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news."
Met Police Chief Superintendent Julia Pendry added: "A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon."
The explosions in Boston took place after the elite race had finished.
Lelisa Desisa of Ethiopa won the men's race with Rita Jeptoo winning the women's event.
Paula Radcliffe, the British women's marathon world record holder, said she was "horrified to hear news of bomb explosion near Boston marathon finish.
"Really hope there are no serious casualties. Situation looks awful, thoughts with everyone. There are some very sick people out there, who would do something like this?"
The Boston race, the world's oldest annual Marathon, is held each year on the United States' Patriot's Day.