Security on New York City mass transit system has been tightened again as police began random bag searches on subway and bus riders following a fresh wave of bombing attacks in London Thursday.

Police Commissioner Ray Kelly told a press conference that police have begun randomly searching subway passengers carrying bags or backpacks, and may conduct similar searches aboard city buses.

"It might slow individuals down, but we will do it in a reasonable way," he said, adding that the random search would be a "systematized approach" to checking bags.

New York Mayor Michael Bloomberg said despite the precautions, New Yorkers should go about their normal lives.

"I want to emphasize that there is still no threats to this city that have been explicitly made or to our subway or bus system, people should go ahead and feel comfortable in using it," he said.

Four small blasts forced the evacuation of three London underground railway stations and a double-decker bus Thursday, but so far there have been reports of only one person injured.

Kelly said the searches will be done before passengers enter the subway, generally before they swipe their MetroCards.

"Ideally it will be before you go through the turnstile, you have a right to turn around and leave," Kelly said. "But we also reserve the right to do those types of searches if in fact someone is already inside the system."

Following London blasts last week, New York police have tightened security on mass transit system, one of the biggest in the nation. These measures include increasing police presence in all transportation hubs, sending an officer on each train during rush hours,and monitoring subway tunnels that cross the rivers 24 hours a day.

Source: CD/Agencies