Officials in the United States have launched an investigation following two explosions near the finish line of the Boston Marathon.
FBI investigators launched a “potential terrorist inquiry” as three people were left dead with upward of 100 more injured.
At least 17 people are critically wounded with injuries including several amputations.
In a televised address following the incident, president Barack Obama vowed to bring those behind the attack to justice.
“We will find out who did this.
“We will find out why they did this,” he said.
“Any responsible individuals, any responsible groups, will feel the full weight of justice.”
The first explosion came at about 14:50 local time on the north side of Boylston Street.
Seconds later another explosion struck the crows approximately 100 metres further back along the course.
British prime minister David Cameron offered his condolences: “The scenes from Boston are shocking and horrific - my thoughts are with all those who have been affected.”
Meanwhile officials in the UK said security around the London Marathon would be reviewed after the incident in Boston.
However, the event will go ahead, after sports minister Hugh Robertson said he was “absolutely confident” it could be kept safe.
London had “enormous experience” of delivering major events and the UK had some of the best security professionals in the world, he told the BBC.
He also said he was “horrified” by the Boston blasts.