London Marathon organisers say the race will go ahead on Sunday despite the death of at least three people in explosions near the finish line of the Boston Marathon yesterday.
Police and other agencies plan to review security plans.
"We will not be cancelling, what we are doing, we are reviewing," London Marathon chief executive Nick Bitel told the BBC.
"We run through the city, when you have an event of any nature, a marathon, a 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."
Chief Superintendent Julia Pendry, the event commander, confirmed that security arrangements would be reviewed following the explosions in Massachusetts.
Hundreds of overseas athletes come to London for the annual marathon which attracts a huge number of spectators along a route which takes in many of the city’s major landmarks.
