Boston - At least two people were killed and 22 wounded when two explosions struck near the finish line of the Boston Marathon on Monday, sparking scenes of panic, police said.
The streets were littered with debris and blood and paramedics raced off with stretchers as police locked down the area, witnesses said. TV footage showed an explosion sending up a white plume of smoke along the sidelines of the race.
Boston police confirmed the toll of dead and wounded on their Twitter feed.
Local radio said the first explosion took place near a sports store and the second went off near a viewing stand. It was not immediately clear whether the explosions were accidental or the result of a terrorist attack.
"There were two bombs that exploded near the finish line in today's Boston Marathon," race organisers said on the event's Facebook page, without providing a source for the information.
"We are working with law enforcement to understand what exactly has happened," it said.
NBC News later reported that "multiple explosive devices" were found in Boston.
US President Barack Obama was notified about the incident, and his administration was in contact with state and local authorities, a White House official said.
Security was stepped up in New York and Washington, sites of the 9/11 attacks. In the Big Apple, police said they were boosting security at hotels and "other prominent locations in the city."
The blasts in Boston rattled US markets, sending the Dow and the S&P 500 down at the close.
Zara Bielkus, a 30-year-old spectator from Boston, said she heard two explosions seconds apart. Police then locked down the area.
"When we heard them, everyone looked at each other and went very quiet, and within a minute police came," she said.
The owner of a hotel near the finish line said there were bloody limbs on his patio in the aftermath of the explosion.
Local media reported a third explosion nearby, but cited police as saying it was a controlled detonation.
The Boston Marathon is one of the biggest annual athletic events held in the United States, with nearly 27 000 racers who must qualify to compete and tens of thousands of spectators.
The race attracts world-class athletes, most of whom would have likely completed the race a couple hours before the blast went off. Footage of the blast showed the marathon time clock at 4:09.
Results on Monday in the 2013 Boston Marathon:
Men
1. Lelisa Desisa (ETH) 2hr 10min 22sec
2. Micah Kogo (KEN) 2:10:27
3. Gebregziabher Gebremariam (ETH) 2:10:28
4. Jason Hartmann (USA) 2:12:12
5. Wesley Korir (KEN) 2:12:30
6. Markos Geneti (ETH) 2:12:44
7. Dickson Chumba (KEN) 2:14:08
8. Jeffrey Hunt (AUS) 2:14:28
9. Daniel Tapia (USA) 2:14:30
10. Craig Leon (USA) 2:14:38
Women
1. Rita Jeptoo (KEN) 2:26:25
2. Meseret Hailu (ETH) 2:26:58
3. Sharon Cherop (KEN) 2:27:01
4. Shalane Flanagan (USA) 2:27:08
5. Tirfi Tsegaye (ETH) 2:28:09
6. Kara Goucher (USA) 2:28:11
7. Madai Perez (MEX) 2:28:59
8. Diane Nukuri-Johnson (BDI) 2:29:54
9. Ana Dulce Felix (POR) 2:30:05
10. Sabrina Mockenhaupt (GER) 2:30:09