Secret Service outside the White House today, after the Boston Marathon explosions (Getty Images)

The Metropolitan Police Department and Metro Transit Police are increasing patrols this afternoon following fatal bombings in Boston. Metro Transit Police officers currently on daytime patrols will stay on through the evening rush hour, while additional units are being called in.

Meanwhile, D.C. Police Chief Cathy Lanier says that the Metropolitan Police Department will be going to a heightened state of alertness, including the deployment of counter-terrorism units, in the wake of the explosions at the finish line of the Boston Marathon.

Metro spokesman Dan Stessel says that officials are in contact with the Joint Terrorism Task Force and federal law enforcement agencies, and that there are no specific or credible threats against the transit system.

Security is also being increased around the White House. U.S. Secret Service agents are clearing Pennsylvania Avenue, and tourists and passers by are being pushed back to Lafayette Park, which remains open.

“Out of an abundance of caution we are increasing the security perimeter around the White House,” Secret Service spokesman George Ogilvie says. Ogilvie would not elaborate on how far the perimeter extends, though traffic on 15th and 17th streets is proceeding as usual.

The Boston Police Department reports 28 injured and two dead in the explosions, the causes for which are still unknown.