**UPDATE: President Barack Obama delievered a statement regarding the deadly Boston explosions today at 6:10 PM ET from the White House press room.
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(Reuters) - Two simultaneous explosions ripped through the crowd at the finish line of the Boston Marathon on Monday, killing two people and injuring dozens on a day when tens of thousands of people pack the streets to watch the world famous race.
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. Other pictures showed blood stains on the ground and several people knocked down.
An hour after the 2:50 p.m. EDT (1850 GMT) blasts in Boston's Copley Square marred the usually joyous end to the marathon, a fire erupted at the John F. Kennedy Presidential Library three miles away, but no one was injured, police said.
Boston Police Commissioner Ed Davis told a news conference the authorities were not certain whether the fire, possibly started by an incendiary device, was related.
Two high-level U.S. law enforcement officials said one or more bombs caused the explosions at the scene of the marathon, which is run annually on the state holiday Patriots' Day.
"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.
The blasts put police on alert in major cities across the United States, including in Washington, D.C. where President Barack Obama was briefed by security personnel.
Three Boston area-hospitals contacted by Reuters reported a total of at least 51 hurt. Some of those may have been hospitalized for treatment from running the marathon. The Boston Globe newspaper reported on Twitter that more than 100 people were hurt, but it did not provide a source for the information.
Ambulances, fire trucks and dozens of police vehicles converged at the finish line, and spectators could be seen crying and consoling each other.
"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.
The two explosions were about 50 to 100 yards (meters) 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.
Of the 23,326 runners who started the race on Monday, 17,584 finished before the blast, marathon officials said. The runners were diverted before officials brought the marathon to a halt.
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 in the air, Mitchell said. People began running and screaming after hearing the noise, Mitchell said.
"Everybody freaked out," Mitchell said.
"Every asset of the Commonwealth (state) of Massachusetts and the federal government is either here or coming here," Governor Deval Patrick told reporters.
The U.S. Federal Bureau of Investigation, Justice Department, Homeland Security Department and other agencies were all lending assistance to the investigation, authorities said.
Obama 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.
Spectators typically line the 26.2 mile race course, with the heaviest crowds near the finish line.
The transit agency shut down all service to the area, citing police activity, and the U.S. Federal Aviation Administration temporarily restricted airspace over the scene, a spokesman said.
The Boston Marathon has been held on Patriot's Day, the third Monday of April, since 1897. The event, which starts in Hopkinton, Massachusetts and ends in Boston's Copley Square, attracts an estimated half-million spectators and some 20,000 participants every year.
The Boston Symphony Orchestra cancelled Monday night's concert and the National Hockey League's Boston Bruins canceled their home game against the Ottawa Senators.
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.
(Reporting by Scott Malone, Tim McLaughlin, Aaron Pressman, Edith Honan, Frank McGurty and Mark Hosenball; Editing by Daniel Trotta and Grant McCool)