The successive blasts went off less than a minute apart shortly before 3 p.m. local time near Copley Square in Boston, according to multiple reports. Though the race winners had apparently crossed the line and left the area two hours before the explosions, runners were still in action at an event.

After the blasts, Boston police blocked headquarters of Boston Marathon, which was located in hotel. Although there is as yet no official explanation as to what caused the two explosions, it appears almost certain to be a terrorist attack after ball bearings were found at the site of the explosions.

Boston police have confirmed that at least two people are dead and 22 injured, though those numbers are expected to rise. The New York Post says emergency services told them at least 12 people are dead with more than twice that number injured in the incident.

Local mass media reports that intelligence officials have said three further explosive devices have been found and were dismantled.

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 Boston’s Copley Square, attracts an estimated half-million spectators and some 20,000 participants every year.

Sunday’s London Marathon faces possible cancellation after horrifying scenes at the finish line of the Boston Marathon.

Scotland Yard have already confirmed that a major security review will take place for the London Marathon ahead of the weekend.

There is already widespread speculation that the race, which attracts nearly 40,000 runners and half a million spectators each year, may have to be cancelled since London is considered a top target for international terrorism.