New York City and Washington, D.C., are tightening their security in wake of the blasts at the Boston Marathon, which have resulted in at least two deaths and 23 injuries.
New York Police Department spokesman Paul Browne said security will be stepped up at hotels and other prominent locations in the city until more about the explosions are known.
New York City Mayor Michael Bloomberg said in a statement that the "NYPD has stepped up security at strategic locations and critical infrastructure, including our subways. Some of the security steps we are taking may be noticeable, including deployment of Critical Response Vehicles and additional police personnel, and others will not be. 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."
Washington's Pennsylvania Avenue, which runs in front of the White House, has been closed to pedestrians. Police at the nation’s capital are also in a heightened level of security.
The Federal Aviation Administration has announced a ground stop for Boston Logan Airport until further notice.
A White House official told NBC News that the president has been notified of the explosions, which occurred roughly during the four-hour mark of the prolific race. “He directed his administration to provide whatever assistance is necessary in the investigation and the response," said the official.
The cause of the blasts, which occurred near the finish line, is not yet known. The Boston Marathon, on its Facebook page, referred to the explosions as “bombs.”
More than half a million spectators gather at the event every year. This year, 26,839 runners were participating in the race, representing 96 countries of citizenship.
British police are saying they are reviewing their security plan for Sunday’s London Marathon.
Nick Bitel, London Marathon’s chief executive said he was “deeply saddened and shocked” by the news, adding “"Our security plan is developed jointly with the Metropolitan Police and we were in contact with them as soon as we heard the news."
Met Police Chief Superintendent Julia Pendry said in a statement that "A security plan is in place for the London Marathon. We will be reviewing our security arrangements in partnership with London Marathon."
UPDATE, 5:25 p.m. ET: New Jersey Transit is currently in a heightened state of alert. Spokesman John Durso, Jr. says in a statement that "While there has been no specific threat made to our transit system at this time, NJ Transit police have deployed both uniformed and plain clothed officers to further patrol our system and keep our customers safe."
UPDATE: 5:39 p.m. ET: Pentagon steps up security. More uniformed police officers have been added both inside and outside the building. A senior defense official tells NBC News that it's nothing more than "prudent planning."