Two explosions hit the Boston Marathon as runners crossed the finish line on Monday, killing at least two people and injuring 23 on a day when tens of thousands of people pack the streets to watch one of the world's best known marathons.
Pictures from the scene showed blood stains on the ground and several people knocked down. Massachusetts General Hospital was treating victims of the explosion in its emergency room but information about their condition was not immediately available, a spokeswoman said.
Police reported at least one explosion and witnesses said there were two, which hit as spectators were cheering on people finishing the Boston Marathon, which was first run in 1897.
Reporters in the media center heard two blasts.
Boston police said two people were killed and 23 injured.
Scores of people, some bloody, wandered the streets around the finish line, though their numbers were thinning as police, some carrying heavy weaponry, tried to clear the area and a pungent smell hung in the air.
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 (15 metres) in the air, Mitchell said. People began running and screaming after hearing the noise, Mitchell said.
"Everybody freaked out," Mitchell said.
Ambulances, fire trucks and dozens of police vehicles converged at the finish line.
U.S. President Barack Obama was notified and directed his administration to provide whatever assistance was necessary, the White House said.
"Blood everywhere, victims carried out on stretchers. I saw someone lose their leg, people are crying," the Boston Globe's Steve Silva reported from the citing, the Globe said on Twitter.
Hundreds of thousands of spectators typically line the 26.2 mile (42.19 km) race course, with the heaviest crowds near the finish line. The blasts occurred more than five hours after the start of the race, at a time when most top athletes were off the course but slower amateur marathoners were still running.
The transit agency shut down all service to the area, citing police activity.
Ambulances arrived on the scene within minutes and runners and spectators could be seen crying and consoling each other.
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.
Earlier on Monday, Ethiopia's Lelisa Desisa and Kenya's Rita Jeptoo won the men's and women's events, continuing African runners' dominance in the sport.
The New York Police Department stepped up security around landmarks in Manhattan, including near prominent hotels, in response reports out of Boston, said Paul Browne, deputy commissioner of the NYPD.
New York police were redeploying counterterrorism vehicles around the city, Browne said.
Reporting by Scott Malone, Tim McLaughlin, Edith Honan, Frank McGurty and Mark Hosenball; Editing by Daniel Trotta and Grant McCool