NYPD officers stand in the Times Square district of New York underneath a news ticker displaying details from fatal explosions in Boston. Reuters
Boston police commissioner Edward Davis speaks about the explosions during a press conference as Massachusetts Gov Deval Patrick, right, looks on in Boston. AP Photo
A man is loaded into an ambulance after he was injured by one of two bombs exploded during the 117th Boston Marathon near Copley Square ...
A Boston police officer clears Boylston Street following an explosion at the finish line of the 2013 Boston Marathon in Boston. AP Photo
A woman is comforted by a man near a triage tent set up for the Boston Marathon after explosions went off at the 117th Boston ...
Medical workers aid injured people at the 2013 Boston Marathon following an explosion in Boston. AP Photo
Helicopters fly near the construction site of One World Trade Center in Manhattan seen from The Heights neighborhood of Jersey City, NJ. New York is ...
Runner John Ounao crying after several explosions rocked the finish of the Boston Marathon in Boston, Massachusetts.AFP
Public safety officials evacuate the scene after several explosions near the finish line of the Boston Marathon in Boston.Reuters
Medical workers aid an injured man at the 2013 Boston Marathon following an explosion in Boston. AP Photo
Two bombs ripped through the crowd at the finish line of the Boston Marathon on Monday, killing three people, causing several amputations and injuring more than 100 in what a White House official said would be handled as an "act of terror."
It was the worst bombing on US soil since security was tightened after the attacks of September 11, 2001, and US President Barack Obama promised to hunt down whoever was responsible for the attack on a day when tens of thousands of spectators packed the streets to watch the world-famous race.
No one has been arrested and the White House official said it would have to be determined whether the attack came from a foreign or domestic source.
Many runners were heading for the finish when a fireball and smoke rose from behind cheering spectators and a row of flags representing the countries of participants, video from the scene showed.
The cheers turned to screams and panic.
"I saw people who looked like they had their legs blown off. There was a lot of blood over their legs. Then people were being pushed in wheelchairs," said Joe Anderson, 33, a fisherman from Pembroke, Massachusetts, who had just run the race holding a large US flag.
Runners continue to run towards the finish line of the Boston Marathon as an explosion erupts near the finish line of the race in Boston. (Reuters/Dan Lampariello)
A Rhode Island state trooper told The New York Times body parts littered the scene.
"These runners just finished and they don't have legs now," Roupen Bastajian said. "So many of them. There are so many people without legs. It's all blood. There's blood everywhere. You got bones, fragments. It's disgusting. It's like a war zone."
Ambulances, fire trucks and dozens of police vehicles converged at the scene, and spectators could be seen crying and consoling each other.
The dead included an 8-year-old boy, the Boston Globe reported, citing two law enforcement sources briefed on the investigation.
A 2-year-old was being treated with a head injury at Boston Children's Hospital, the hospital said in a statement.
"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.
Boston Police Commissioner Ed Davis called them "powerful devices."
Davis told a news conference at least three people died and Massachusetts Governor Deval Patrick said more than 100 people were wounded.
FBI Boston Special Agent In Charge Richard DesLauriers declined at the same news conference to comment on media reports that police found unexploded devices.
The US Federal Bureau of Investigation took the lead on the investigation with help from several other federal, state and local law enforcement agencies.
The two explosions at 2.50pm (1850 GMT) were about 50 to 100 yards (metres) apart as runners crossed the finish line with a timer showing 4 hours and 9 minutes, some 9 minutes faster than the average finish time, as reported by Runner's World magazine.
Spectators typically line the 26.2 mile (42.19 km) race course, with the heaviest crowds near the finish line.
Injured people and debris lie on the sidewalk near the Boston Marathon finish line following an explosion in Boston. (AP)
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 50 feet (15 metres) in the air, Mitchell said. People began running and screaming after hearing the noise, Mitchell said.
"Everybody freaked out," Mitchell said.
'Act of terror'
Without knowing who perpetrated the attack, the White House said it was handling the incident as "an act of terror."
"Any event with multiple explosive devices - as this appears to be - is clearly an act of terror, and will be approached as an act of terror," a White House official said.
Blood from victims covers the sidewalk on Boylston Street, at the site of an explosion during the 2013 Boston Marathon in Boston. (AP)
The annual Boston Marathon, held since 1897, 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 respectively, continuing African runners' dominance in the sport.
The Boston Symphony Orchestra canceled Monday night's concert and the National Hockey League's Boston Bruins canceled their home game against the Ottawa Senators. The Boston Red Sox had completed their Major League Baseball game at Fenway Park before the explosions.
Indians in Boston can contact: 202-939-7000