London Marathon organisers and Metropolitan police are now reviewing security plans for Sunday’s race in the capital, and the Met is also likely to look again at security plans for Baroness Thatcher’s funeral tomorrow.
Dramatic footage showed runners toppled by the double explosion, followed by scenes of smoke and confusion as medics began treating bloodied victims.
Boston police said a third incident occurred later at the city’s JFK Library and was under investigation.
Of the 23,326 runners who started the race, 17,584 finished before the blast at 2.50pm local time (7.50pm GMT). The race – one of the biggest sporting events in the US – was taking place on the public holiday Patriots’ Day.
Late last night President Barack Obama said it was not known who caused the blasts, but any group found to be responsible would feel the “full weight of justice”.
“We will find out who did this and we will find them accountable,” he said, while warning Americans not to jump to conclusions about who carried out the attacks.