"We will be reviewing our security arrangements in partnership with the London Marathon," Metropolitan Police chief superintendent Julia Pendry said in a statement.
In Boston, loud blasts were heard and large puffs of smoke could be seen as thousands of people packed the streets to watch runners cross the finish line.
The explosions appeared to go off near where spectators stood behind roadside barriers although one runner was seen collapsing to the ground as he heard the blasts.
"We are deeply saddened and shocked by the news from Boston," London Marathon chief executive Nick Bitel said in a statement.
"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," said Bitel before adding the London race would definitely go ahead.
