Britons were tonight feared to be among the injured after two explosions rocked the Boston Marathon killing two and injuring at least 23 people.
Horrifying images of injured spectators and blood-splattered pavements were caught as television cameras broadcast footage of the blasts.
Participants were also seen lying on the ground as the two explosions tore through the finish line, sending smoke and debris soaring into the air.
Emergency workers ripped away fencing as others carried injured men and women from the scene.
Cheryl Fiandaca, the Bureau Chief of the Boston Police Department, tweeted that two people had died and 23 people were injured.
She wrote: “Updates to follow. Please clear area around marathon finish line.”
The blast, which came four hours into the race, comes just six days before the London Marathon on Sunday, prompting a “review” of security ahead of the event.
Chief Superintendent Julia Pendry, of the Met Police, the event commander for the event in the capital, 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.”
Pictures 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.”
A message on the Boston Marathon Facebook page stated: “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.”