SECURITY arrangements are being stepped up at this weekend's London Marathon following the bombings that killed two people and injured more than 100 at the Boston Marathon.

There were fears Britons were among the injured when two bombs exploded at the finishing line of the event. Seven athletes on the list of runners had given Glasgow as their home city.

A total of 374 from the UK were on the list of entrants among nearly 20,000 participants. Tens of thousands of spectators were lining the streets when the bombs exploded.

A Foreign Office spokesman said: "We are aware of the incident and we are trying to gather information."

Nick Bitel, London Marathon chief executive, said: "We are deeply saddened and shocked by the news from Boston.

"Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news."

Metropolitan Police Chief Superintendent Julia Pendry said: "A security plan is in place for the London Marathon."

Images from Boston showed horrifying scenes of injured spectators and blood-splattered pavements.

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.

Casualties were seen lying on the pavement in Boylston Street – the main road through Boston – and debris blowing around them.

One photograph showed athletes running down the street at the moment an explosion created a fireball, sending smoke into the air.

Boston Police Commissioner Ed Davis said there had also been a third explosion at the JFK Library in the city, but this was later reported to be a fire unrelated to the blasts.

Two more explosive devices were reportedly found at the marathon site and dismantled.

After the blasts, which happened about 7.50pm British time, the US Federal Aviation Administration placed temporary flight restrictions in the airspace near the event's finish line.

The FBI was involved in the investigation into the explosions and police were later reported to be holding "a person of interest" in connection with the blasts.

However, this was later denied by the police.

Foreign Secretary William Hague said on the Twitter networking site: "Appalled by news of explosion at Boston Marathon. My thoughts are with everyone affected by it and all those waiting for news."

Mike Mitchell, of Vancouver, Canada, a runner who had finished the race, said he was looking back at the finish line and saw a "massive explosion".

Smoke rose 50 feet and people began running and screaming after hearing the noise, Mr Mitchell said.

He added: "Everybody freaked out."

Ambulances, fire trucks and dozens of police vehicles converged at the finish line.

American President Barack Obama directed his administration to provide whatever assistance was necessary.

Hundreds of thousands of spectators typically line the 26.2-mile race course, with the heaviest crowds near the finish line.

The blasts happened more than five hours after the start of the race, at a time when most top athletes were off the course but slower amateur runners were still finishing.

Athlete Andrew Lemoncello, from St Andrews, said he and two other athletes, Ian Torrence and Scott Smith were "safe and in their hotel".

English Olympic athlete Ross Murray, who ran the one mile invitational at the event, went on Twitter to say he was "absolutely fine".

The 22-year-old said: "Just so everyone knows, I'm okay and well. Wasn't around the bombs."

"In the hotel, most seem okay but a few seem shaken up. Witnessed a few very relieved families meeting up."

The Boston Marathon has been held on Patriots Day, the third Monday of April, since 1897.

The event, which starts in Hopkinton, Massachusetts and ends in Boston's Copley Square, attracts about 500,000 spectators and some 20,000 participants every year.

The New York Police Department stepped up security around landmarks in Manhattan, including near prominent hotels, in response to reports out of Boston, said Paul Browne, NYPD deputy commissioner.

New York police were redeploying counterterrorism vehicles around the city, Mr Browne said.