Two people were killed and 23 others injured after two explosions struck the Boston Marathon as runners crossed the finish line on Monday, Boston police said.
Massachusetts General Hospital was treating 19 victims of the explosion in its emergency room but information about their condition was not immediately available, a spokeswoman said.
Tens of thousands of people pack the streets to watch one of the world's best known marathons
Pictures from the scene on Monday showed blood stains on the ground and several people knocked down.
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 centre heard two blasts.
"There was an explosion. Police, fire and EMS [emergency medical services] are on the scene, we have no indication of how many people are injured," a spokesperson for the Boston police department said.
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 15 metres in the air, Mitchell said.
People began running and screaming after hearing the noise, Mitchell said. "Everybody freaked out," Mitchell said.
Television images showed ambulances, fire trucks and dozens of police vehicles near the finish line.
Hundreds of thousands of spectators typically line the 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.
President Barack Obama directed his administration on Monday to aid Boston authorities in investigating the cause of what local reports said were twin explosions that rocked the finish line of the Boston Marathon.
A White House official said Obama had been notified of the "incident in Boston".
"His administration is in contact with state and local authorities. He directed his administration to provide whatever assistance is necessary in the investigation and response," the official said.
Boston police said an explosion occurred at the John F Kennedy presidential library and museum in Boston after other blasts in the city near the finish line of the Boston Marathon. Police spokesperson Neva Coakley said "there has been another explosion at JFK".
The US Federal Aviation Administration said it has placed temporary flight restrictions in the airspace over the site of two explosions near the finish line of the Boston Marathon. The restrictions will not affect commercial air operations at Boston's Logan Airport, the FAA said.
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. – Reuters