Two explosions near the finishing line of the Boston Marathon which killed three people including an eight-year-old-boy are being probed by the FBI in in a “potential terrorist inquiry”.
Around 140 people were injured in the blasts, at least 17 of them critically. Officials say the injuries include several amputations.
According to media reports in Boston, the eight-year-old boy killed had gone out to hug his dad after he crossed the finish line.
The dad walked on and the boy went back to join his mother and sister, at which point the bomb went off.
Reports suggest his sister’s leg was blown off and his mother was badly injured.
The first explosion came at about 2.50pm local time (6.50pm GMT) on the north side of Boylston Street, about two hours after the winners crossed the line.
Then seconds later another explosion tore into the crowd further away from the finishing line, between 50 and 100 metres from the first blast.
TV footage showed bloodied runners and spectators being treated at the scene alongside blood-splattered pavements and the road strewn with debris.
About 375 Britons were listed as taking part, but there has no been no confirmation of any British casualties.
The FBI has taken over co-ordination of what it described as a "potential terrorist inquiry". Boston police say officers are working around the clock and all leave has been cancelled.
In a televised address President Barack Obama said: “We will find out who did this. We'll find out why they did this.
“Any responsible individuals, any responsible groups, will feel the full weight of justice.”
Anti-terrorism vehicles were put in place around major landmarks while extra protection was reported at the White House.
Police have vowed to review security around the London Marathon following the bomb attack but organisers insist the Sunday race will go on.
Nick Bitel, London Marathon Chief Executive, 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.”
Met Police Chief Superintendent Julia Pendry said: “A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon.”
Twitter users have paid tribute to those affected by the blasts, creating a hashtag #prayforboston.
User @ hereforbiebahh wrote: “RIP to all the souls who were affected by the Boston bombing. you'll be in my thoughts and prayers. Heaven gained more angels #PrayForBoston”.
User @Jamimuuy wrote: “You watch movie = you get shot. You go to school = you get shot. You run a marathon = you get bombed. I hate this world. #prayforboston”.
One Direction’s Harry Styles said: “Just heard the news. So terribly sad. thoughts with everyone in Boston .x”
And Australian model Miranda Kerr wrote: “Sending love and prayers to all those affected by the tragic event in Boston”.
Meanwhile Taylor Swift tweeted: “Sending all of my love to Boston after a day of sadness and confusion and not knowing what to say. I just don't understand.”