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.
British Prime Minister David Cameron wrote on Twitter that footage from the blasts was “shocking and horrific”, and said that his thoughts were with those affected.
The harrowing scenes of injured spectators 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.