WASHINGTON, July 23: Public transit systems across the United States have tightened security following the second series of bombings in London this week.

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

In New York – the first US city to start randomly checking bags at trains and buses – police began searching passengers bringing bags into the city’s subway system. Individuals who refuse to comply are turned away from the station.

New York City Mayor Michael Bloomberg 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.

In Washington, policemen and sniffing-dogs were deployed at all major metro stations. At some places they were seen sweeping metro stations for explosives or weapons.

But unlike New York, Washington metro police are not yet searching riders’ bags. Metro Transit Police chief Polly Hanson told reporters in Washington that Metro is studying the idea of bag searches. They’ll watch how it works in New York.

Ms Hanson said maintenance work has been cancelled overnight to allow for the security sweep. But she wouldn’t discuss the details.