BREAKING NEWS: Two dead, dozens injured in Boston Marathon explosion

Two people have died and at least 51 are injured after two explosions hit the Boston Marathon as runners crossed the finish line today.

Boston police also reported another explosion at the John F Kennedy Presidential Library, which is nearly 5km 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.

The Boston Police Department said the incident at the library appeared to be fire related.

The New York Post is reporting 12 people have died, however this has not been confirmed. The website also reports officials have identified a suspect, who is being guarded in hospitals.

CNN have reported figures from three hospitals place the number of injured as high as 51.

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.

"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.

Around 45 New Zealanders are believed to have been competing in the event today.

Former New Zealand MP Laila Harre told TV ONE's Breakfast she had just finished running the marathon when she heard the explosions.

"It was a very, very loud and deep boom, nothing like i have ever heard before.

"My immediate thought was that it was something terrible."

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 15m in the air and people began running and screaming after hearing the noise, Mitchell said.

"Everybody freaked out," Mitchell said.

Massachusetts General Hospital was treating 19 victims of the explosion in its emergency room, six of them in critical condition, a spokeswoman 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.