Security has been heightened on transit systems in the United States following the latest bombings in London.

An increased police presence was visible Friday in and around transit systems in cities including Boston, New York and Washington.

New York City police began randomly searching bags being taken into the city's subway system. Individuals who refuse to comply are being turned away from the station.

Police are also searching bags on New York City buses and commuter trains.

New York City Mayor Michael Bloomberg Thursday called the searches a little intrusive, but necessary. Police Commissioner Raymond Kelly said the police will not target people because of their ethnic background.

Civil liberty groups have expressed concern that searches could lead to racial profiling and violate an individual's rights.

Some information for this report provided by AP.