Three people were killed and more than 140 injured as two blasts brought death and destruction to the finishing line of the Boston Marathon.
Horrifying images of injured spectators and blood-splattered pavements were caught as television cameras broadcast footage of the explosions.
Participants were also seen lying on the ground as the impact tore through the finish line, sending smoke and debris soaring into the air.
Britons were feared to be among those injured after explosions ripped through the finish line of the Boston Marathon.
The FBI said it was leading a "potential terrorism investigation" by the city's joint terrorism taskforce into the twin explosions. But reports of a suspect being held at a city hospital were dismissed by Ed Davis, Boston's Police Commissioner, who told a press conference that no suspect had been arrested.
In a televised address, US President Barack Obama said those responsible will feel the "full weight of justice".
Speaking from the White House, he paid tribute to those who helped wounded runners and spectators, saying: "The American people will say a prayer for Boston tonight, and Michelle and I send our deepest thoughts and prayers to the families of the victims in the wake of this senseless loss. We don't yet have all the answers but we do know that multiple people have been wounded, some gravely, in explosions at the Boston Marathon."
Mr Obama added: "We still do not know who did this, or why. And people shouldn't jump to conclusions before we have all the facts. But, make no mistake, we will get to the bottom of this, and we will find out who did this, we will find out why they did this. Any responsible individuals, any responsible groups, will feel the full weight of justice."
Mr Davis said there had been two "simultaneous explosions" near the finish line of the 26.2-mile event around four hours into the race. He told a press conference that "at least three people" were dead but that exact figures on the number injured would be released later. Hospitals reported at least 144 injured, at least 17 of them critically.
Sources said an eight-year-old boy was among those killed, while his mother and sister were injured.
"This cowardly act will not be taken within stride, we will turn every rock over to find the people responsible for this," Mr Davis said.
Britons were feared to be among those injured after explosions ripped through the finish line of the Boston Marathon.
Television pictures showed bloodied spectators and runners lying on the ground, emergency workers ripping away fencing and others carrying injured men and women from the scene.
Pictures also emerged on Twitter showing casualties lying on the pavement on Boylston Street - the main road through the east coast city - and debris blowing around them. Another photograph appeared to show participants running down the street at the moment an explosion created a fireball, sending smoke into the air.
A list of competitors on the Boston Marathon website showed hundreds of British runners were expected to line up for the race. A Foreign Office spokesman said: "We are aware of the incident and we are trying to gather information." Foreign Secretary William Hague tweeted: "Appalled by news of explosion at Boston marathon. My thoughts are with everyone affected by it and all those waiting for news."
Prime Minister David Cameron wrote on Twitter: "The scenes from Boston are shocking and horrific - my thoughts are with all those who have been affected."
The blasts struck six days before thousands of athletes and fun-runners will take part in the London Marathon next Sunday. Chief Superintendent Julia Pendry, the event commander, said security arrangements will be reviewed following the explosions in Massachusetts. She said: "A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon."
Labour leader Ed Miliband wrote on Twitter: "Appalling news from Boston. All of my thoughts are with the casualties and their families."
Runner Darren Foy, 40, from Southampton and his wife Sandra and their two children missed the explosions by just 30 minutes after he finished the marathon in three and half hours.
Speaking from the city, the chartered surveyor said: "We were on our way home when we heard something had happened and I was getting messages like 'are you Ok?' which I never get. It's all quite shocking really. We got home and we looked at the BBC online and saw there had been explosions but we are OK because I finished in three and a half hours and we were on the bus when it happened. We are staying five miles outside and I'm not intending to go back into the city for a few days."
Security for the London Marathon and Baroness Thatcher's funeral will be under scrutiny in the aftermath of the devastating blasts. The Metropolitan Police said they would review the policing of the race on Sunday, while former Home Secretary John Reid said the incident will also have ramifications for Lady Thatcher's funeral on Wednesday.
