The U.S. and Britain are tightening security in major cities in the wake of deadly explosions at the Boston Marathon on Monday afternoon, local time.

The Los Angeles Police Department ordered police officers to beef up security at major public buildings and areas, including key sports stadiums.

The attack that has been classified by the FBI as a terrorist act brought some sporting events to a halt. The National Hockey League's Boston Bruins called off a home game scheduled on Monday, and the National Basketball League's Boston Celtics postponed their Tuesday match.

New York police deployed rapid response forces to public buildings and hotels. Police officers in other major U.S. cities are also on tighter guard.

The London Marathon will be held on Sunday as scheduled and British police are moving to strengthen security for the race.