The Metropolitan Police say they will be reviewing their security procedures following two explosions at the Boston Marathon in Massachusetts.
The blasts struck six days before thousands of athletes and fun-runners take part in the London Marathon.
Chief Superintendent Julia Pendry, the event commander, said security arrangements would be reviewed ahead of the event next Sunday.
She said: "A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon."
Nick Bitel, the chief executive of London Marathon, said: "We are deeply saddened and shocked by the news from Boston.
"Our immediate thoughts are with the people there and their families. 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."