Security is being reviewed for the London marathon on Sunday after two bombs exploded killing three people and injuring more than 130 at the end of the Boston race.

FBI special agent Richard DesLauriers, who is in charge of the investigation, said the blasts close to the finishing line of the  Boston marathon were being treated as a “potential terrorist” attack.

The twin explosions put police on alert in major cities across the US, including in Washington DC and New York City, sites of the September 11 attacks in 2001.

There was heavy security around the area of the explosions and outside the hospitals where the injured were being treated. A no-fly zone was in place above central Boston.

The Foreign & Commonwealth Office warned of possible travel disruption in the wake of the explosions at 2.50pm local time yesterday.

“At least two explosions took place close to the finishing line of the Boston marathon, causing many injuries,” the FCO said.

“You should exercise vigilance and caution, monitoring local media and following the advice of local authorities. If in the affected area you should let family and friends know you are safe. Additional information can be obtained through the Massachusetts Emergency Management Agency.”

Boston police set up an emergency hotline for friends and relatives: 001 617 635 4500.

A spokesman for the Fairmount Copley Plaza said yesterday: “An explosion was reported at the finish line of the Boston marathon, two blocks away from the hotel.

“At this time, only guests and colleagues are being allowed into the hotel, and per the direction of the Boston police those already in the building are encouraged to stay inside.”

A room was set up for people to relax and get in touch with loved ones.

“All hotel colleagues are safe and our thoughts are with those affected by this tragic incident,” the spokesman added.

The travel industry was among the first to express its concern and disgust at what happened in Boston.

Cheapflights has an office in Boston and UK employees tweeted they hoped their US colleague was safe.

Travel Counsellors’ chairman David Speakman said the agency had already got a list of customers in Boston and was checking to make sure all were well.

Abta chairman and Advantage Travel Centres chief executive John McEwan said: “Dreadful news coming from Boston with bombs set off at the marathon. My thoughts are with everyone involved.”

There are 2,000 people due to fly to Boston from Heathrow on six flights today.  British Airways is allowing passengers to switch US destinations over the next three days.