Two explosions at the finish line of the Boston Marathon that killed three people and left as many as 140 injured are being probed by the FBI in a "potential terrorist inquiry."
One of those killed in the blast is believed to be an eight-year-old boy.
Horrifying images of injured spectators and blood-splattered pavements were shown as television cameras broadcast footage of the blasts.
At least 17 people are critically injured, with a number needing amputations, reported the BBC.
An explosion rips through the finish line at the Boston Marathon
Dr Natalie Stavas, who works at Boston Children’s Hospital, told the New York Times how she was nearing the finish line when she felt the blast, which came around three hours after the winners had crossed the finish line.
“The police were trying to keep us back, but I told them that I was a physician and they let me through,” she said.
Footage of blood spattered pavements was broadcast
She described the serious injuries of one women she gave CPR to, saying "her legs were pretty much gone.”
Stavas said she applied a tourniquet to one man’s leg with someone’s belt after he had lost his foot. “He was likely in shock,” she said. “He was saying, ‘I’m OK'"
Over 100 people are believed to have been injured
Participants were seen lying on the ground as the two explosions tore through the finish line, sending smoke and debris soaring into the air.
The headquarters for the race was locked down after the blasts and a Boston Marathon spokesman told Reuters that no one would be allowed in or out of the building.
Later at a press conference, Boston police commissioner Ed Davis said there were "simultaneous explosions" that resulted in "multiple casualties".
Davis also said that a third incident had occurred at JFK library in the city but this was later confirmed as being unrelated.
There had been reports of a suspect being held at a city hospital were dismissed by Ed Davis, Boston's Police Commissioner, who told a press conference this morning that no suspect had been arrested.
Other cities soon responded to the events including New York where authorities put anti-terrorism vehicles in place around major landmarks while the White House was reported as receiving extra protection.
Pictures emerged on Twitter showing casualties lying on the pavement on Boylston Street in Boston - 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.
The finishing line was packed with spectators
Massachusetts governor Deval Patrick espressed his horror at the explosions. In a statement he said: "This is a horrific day in Boston. My thoughts and prayers are with those who have been injured. I have been in touch with the President, Mayor Menino and our public safety leaders.
"Our focus is on making sure that the area around Copley Square is safe and secured. I am asking everyone to stay away from Copley Square and let the first responders do their jobs.”
It's not clear whether or not there have been any British casualties although a list of competitors on the Boston Marathon website showed that 347 British runners were expected to line up for the race which involved 25,000 participants.
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.
"It's such a soft target. There are hundreds of thousands spectators on the streets and 27,000 runners, so we got off lightly."
Emergency workers at the scene after the explosions
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."
And the British ambassador to the United States added: "Awful news of explosions in Boston. Thoughts are with those injured or killed, and their families. Our team in touch with MA [Massachusetts] authorities."
Shadow foreign secretary Douglas Alexander also expressed his concern via Twitter, saying: "Horrified to hear reports of explosions at the Boston Marathon. My thoughts are with all those affected."
Labour leader Ed Miliband, who taught at Harvard University in Massachusetts, commented: "Appalling news from Boston. All of my thoughts are with the casualties and their families."
With the London Marathon just days away in which thousands of athletes and fun runners will be on the streets of the capital, the Metropolitan Police said it would be looking at security arrangements following the events in Boston.
"A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon," said chief superintendant Julia Pendry.
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.
The scene at the finish line immediately after one of the blasts
"Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news."
A video of one of the explosions going off shows a bright orange blast and runners falling to the ground.
Footage from the aftermath of the incident shows much confusion with emergency services rushing to help the injured and taking bloodied spectators to a medical tent. Many ambulances could then be seen arriving to rush casualties to hospital.
"There are a lot of people down," said one runner quoted by AP news agency.
ABC Eyewitness News tweeted:
The explosions came around three hours after the winners had crossed the line.
The organisers of the marathon issued a statement on its facebook page, which said: "There were two bombs that exploded near the finish line in today's Boston Marathon. We are working with law enforcement to understand what exactly has happened."
Vice president Joe Biden, who was at a conference on gun control, said: "Apparently there has been a bombing. Our prayers are with those people in Boston who have suffered injuries. I don’t know how many there are."
The race was being held on Patriots' Day, celebrated as an official holiday in Massachusetts and Maine, which honours the first military actions - the Battles of Lexington and Concord - against the British at the start of the War of Independence, on 19 April 1775.
There was a 26-second period of silence before the race began to honour the children killed in the Sandy Hook Elementary School shootinng in Newton, Connecticut last year. As runners passed mile 26, they were met by a marker with the Newtown seal in another display of dedication to those who were gunned down in the Connecticut hamlet.