Cherie Falgoust was waiting for her husband, who was running the race.

"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."

Runners who had not finished the race were diverted straight down Commonwealth Avenue and into a family meeting area, according to an emergency plan that had been in place.

In Washington, shortly after the explosions, the presidential Secret Service shut down Pennsylvania Avenue outside the White House, cordoning off the area with yellow police tape. Several Secret Service patrol cars also blocked off the entry points to the road.

Attorney General Eric Holder directed the full resources of the Justice Department be deployed to investigate and a department official said Holder has spoken with FBI Director Robert Mueller. The official said the US attorney for Massachusetts's office was coordinating the Justice Department's response with the FBI and other federal, state and local law enforcement authorities.

Cities worldwide stepped up security following the explosions.

In Britain, police said they were reviewing security plans for Sunday's London Marathon, the next major international marathon. Thousands of people compete in the London Marathon every year, thronging the city's streets. London is also considered a top target for international terrorists.

A London Metropolitan Police spokesman confirmed Monday that police are working with marathon officials to review security plans for Sunday's event. The London race's chief executive, Nick Bitel, expressed shock and sadness about the situation in Boston, saying "it is a very sad day for athletics and for our friends in marathon running."

In New York City, police spokesman Paul Browne said that critical response teams are deployed around the city. Officials were stepping up security at hotels and other prominent locations.

Police at three major Los Angeles area airports, including Los Angeles International Airport, were in a "heightened state of vigilance," with increased patrols to make it visible that more police were on duty Monday, said Chief of Airport Police Patrick Gannon.

Race day got started with 26 seconds of silence in honor of the victims of the December school massacre in Connecticut. A little more than 2 hours later, the lead runners passed the Mile 26 marker, which was decorated with the Newtown, Connecticut, seal and dedicated to the memory of those killed there.

Boston Athletic Association president Joanne Flaminio previously said there was "special significance" to the fact that the race is 26.2 miles long and 26 people died at Sandy Hook Elementary school.

The annual marathon attracts more than 500,000 spectators. It takes place on Patriot's Day, a state holiday that celebrates the evacuation of Boston by the British in the American Revolution.

Spectator Cherie Falgoust was waiting for her husband, who was running the race.

"I was expecting my husband any minute," she said. "I don't know what this building is ... it just blew. Just a big bomb, a loud boom, and then glass everywhere. Something hit my head. I don't know what it was. I just ducked."

Associated Press writers Jimmy Golen, Jay Lindsay, Steve LeBlanc and Meghan Barr in Boston and Lara Jakes and Eileen Sullivan in Washington contributed to this report.

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At least 2 dead, 100 injured in Boston explosions