Boston police also reported another explosion at the John F Kennedy Presidential Library, which is three miles from the marathon's finishing line.
"We are not certain that these incidents are related, but we are treating them as if they are," Boston police commissioner Ed Davis told a news conference.
Boston police said earlier two people were killed and 23 injured in the explosions on the marathon scene, while the Boston Globe later reported more than 100 were being treated in hospitals.
"It sounded like a sonic boom. I haven't stopped shaking yet," said Melissa Stanley, who watched her daughter cross the finish line four minutes before the explosions.
A fireball rose from behind spectators and a row of flags, video posted on the New York Post website showed.
Other pictures showed blood stains on the ground and several people knocked down.
Massachusetts General Hospital was treating 19 victims of the explosion in its emergency room, six of them in critical condition, a spokesperson said.
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.
Witnesses said two explosions 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.
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.
Ambulances, fire trucks and dozens of police vehicles converged at the finish line.
US President Barack Obama was notified and directed his administration to provide whatever assistance was necessary, the White House said.
Obama was being briefed by Homeland security adviser Lisa Monaco and other staff, the White House said. Authorities tightened security in Washington and New York.
"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 scene, the Globe said on Twitter.
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.
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