Law enforcement at cities across the country are at a heightened level of security following the explosions in Boston.
In New York City, police cars were stationed around Times Square.
"We're stepping up security at hotels and other prominent locations in the city through deployment of the NYPD's critical response vehicles until more about the explosion is learned," Paul J Browne, New York Police Department Deputy Commissioner said in a statement.
Mayor Michael Bloomberg said the city's investments in counter-terrorism infrastructure were being mobilized.
"We have 1,000 members of the NYPD assigned to counter-terrorism duties, and they - along with the entire NYPD and the investments we have made in counter-terrorism infrastructure - are being fully mobilized to protect our city," he said in a statement.
In Washington, D.C., Mayor Vincent Grey reported the city had received no specific threats, but a Metropolitan Police spokesman said their police force was on a "heightened level of security."
In front of the White House, pedestrians were pushed off the street.
Organizers for the London Marathon, which will be held Sunday, said security for the race would be reviewed.
"A security plan is in place for the London Marathon," said Nick Bitel, London Marathon Chief Executive, in a statement. "We will be reviewing our security arrangements in partnership with London Marathon."
Police in Los Angeles, Las Vegas, San Diego, Detroit, Atlanta and Seattle were also at a heightened level of security, according to the Associated Press.