As the horror of explosions near the finish line of the nation's most famous marathon sank in, government officials mobilized to respond on Monday afternoon.
FBI director Robert Mueller briefed President Barack Obama by telephone, the White House said.
All off duty-police officers in Boston were told to report to their districts, CNN affiliate WHDH reported, and the Red Cross sent additional blood products to Boston-area hospitals, spokeswoman Anne Marie Borrego said.
Aviation authorities banned planes from flying over of the site of the explosion, the Federal Aviation Administration said. The temporary flight restriction came at the request of law enforcement and originally extended three nautical miles in all directions before being reduced to two. It reaches up to a height of 3,000 feet, FAA spokeswoman Arlene Salac said.
The agency ordered a brief ground stop at Boston's Logan airport but it did not last long.
And law enforcement authorities from Atlanta to San Francisco and Seattle said they were on heightened alert.
New York's Mayor Michael Bloomberg said the city had 1,000 officers assigned to counterterrorism duties, "and they -- along with the entire NYPD and the investments we have made in counterterrorism infrastructure -- are being fully mobilized to protect our city."
In Miami, police said they were increasing security at critical infrastructure sites, while the Los Angeles Police Department restated its advice to the public: "If you see something, say something."
Secretary of State John Kerry -- a former Massachusetts senator with deep ties to Boston -- found out about the blasts when he was on his way to Chicago on his way home from a 10-day foreign trip, and called his daughter Vanessa, who has run the Boston Marathon in the past, a senior State Department official said.
Kerry himself often fires the starting gun for the wheelchair race at the marathon.
Attorney General Eric Holder instructed the Justice Department to deploy its "full resources" to the investigation into the blasts that killed at least two people and left dozens injured.
He spoke to FBI director Mueller, and to the U.S. Attorney for the District of Massachusetts, Carmen Ortiz. Her office is coordinating responses from the Justice Department, the FBI, the Bureau of Alcohol, Tobacco and Firearms and other law enforcement officials.
Homeland Security Secretary Janet Napolitano was briefed, and she ordered her department to provide "whatever assistance is necessary" to state and local authorities, an administration official said.
The House of Representatives announced it would hold a minute's silence to honor the victims of the explosions.
London's Metropolitan Police said they were reviewing security arrangements for the London Marathon, which is due to take place on Sunday.
The race's chief executive, Nick Bitel, said organizers were "deeply saddened and shocked," and called it "a very sad day for athletics and for our friends and colleagues in marathon running."
British Foreign Secretary William Hague said he was "appalled" by the explosion, while Canada's Prime Minister Stephen Harper said: "It is truly a sad day when an event as inspiring as the Boston Marathon is clouded by such senseless violence."
Major League Baseball senior public relations executive Pat Courtney said the League's "thoughts and prayers are with everyone affected by this horrible occurrence."
The Red Sox were playing in Boston on Monday but the game was over before the terror attack.