At least three people are reported dead and more than 100 wounded after two explosions at the finish line of the Boston marathon.
Another explosion has since occurred at the JFK Library.
CNN producer Matt Frucci said the first two explosions were about 10 seconds apart.
TV helicopter footage showed blood staining the pavement in the Back Bay area.
The explosions occurred about three hours after the winners crossed the line on Monday as watching crowds cheered late runners at the finish line.
Stragglers heading for the finish line were rerouted away from the smoking site of the blasts.
The Eastern Massachusetts branch of the Red Cross has set up a disaster response centre in the area.
The transit agency has shut down all service to the area.
The Boston Globe newspaper reported that more than 100 people were hurt.
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 Copley Square, attracts an estimated 500,000 spectators and some 20,000 participants every year.
Police in New York City have increased security around landmarks in Manhattan in response to the explosions in Boston.
Deputy Commissioner Paul Browne said the NYPD was re-deploying counter-terrorism vehicles around the city.
Word "terrorism" not used by Obama
President Barack Obama promised the full use of federal resources to find out who was to blame for what he called the "senseless loss of life".
"We still do not know who did this or why. And people should not jump to conclusions before we have all the facts."
The BBC reports that the president did not use use the word "terrorism" and given he will face criticism for not using the word, it is certainly deliberate and may be significant.