Passengers on New York buses, subways, and ferries will be subject to “random” searches of their bags, briefcases, and backpacks by police, beginning this morning.

Mayor Bloomberg and Police Commissioner Raymond Kelly announced the search policy yesterday morning, just hours after a second round of bombings in London’s buses and subways.

The announcement drew immediate criticism from civil libertarians, who said the searches would violate New Yorkers’ Fourth Amendment rights to be free of unreasonable searches. The mayor and the police commissioner, however, described it as a step that will increase security against terrorists.

“We’re constantly, always changing the things we do,” Mr. Bloomberg said as he announced the newest tactic in the city’s counterterrorism arsenal. “If we did everything Day 1, terrorists would know exactly how to beat our security systems. That would be totally irresponsible. And this is not the last change in our tactics. We will be changing our tactics all the time, both the presence you see on the subways, on the buses, on the streets, and those that you don’t see.”

In closed-door security meetings since the terrorist attacks of 2001, New York City officials have been discussing the idea of pulling aside commuters and checking the contents of their backpacks and briefcases.

Just last month, before the bombings in London, the Metropolitan Transportation Authority and the Police Department conferred about the strategy, which is already in use in Boston, and which has been discussed in Washington, D.C. The concept gained steam after terrorists set off bombs in the Underground and on a city bus in London, killing dozens of commuters.

Yesterday morning, when news broke that another round of explosions had rocked the London Underground, Mr. Bloomberg consulted the city’s Law Department, which assured him that random searches were legal. Mr. Bloomberg quickly gave Mr. Kelly the go-ahead to start the searches, which will cost the city about $2 million a week.

The MTA simultaneously decided to implement a policy of random searches on Long Island Rail Road and Metro North trains.

The MTA and Police Department searches will begin before 7 a.m. Friday.

Mr. Kelly said the searches would be “random,” but he and other police officials said that officers would search only people carrying cumbersome containers or backpacks. They also said they would pull aside passengers wearing bulky coats inappropriate for the high summer temperatures and those who appeared to be nervous.

“No racial profiling will be allowed,” the police commissioner said. “It’s against our policies.”

A police spokesman, Paul Browne, said although the checkpoints are being established to prevent would-be terrorists from transporting explosives into the subways, buses, and ferries, police officers would not ignore non-explosive illegal materials they find in people’s bags. He said, for example, that officers would arrest subway riders who are found to be carrying guns or drugs.

People have a right to refuse a search, but if they do not consent, they will not be permitted to enter the transit system.

Despite assurances from the Police Department, the mayor, and the governor, civil libertarians yesterday were quick to question whether random searches infringed on New Yorkers’ constitutional rights.

“We are entitled to move freely around the city without being worried about being searched by police,” the executive director of the New York Civil Liberties Union, Donna Lieberman, said. “The NYPD can and should investigate any suspicious activity, but the Fourth Amendment prohibits police from conducting searches where there is no suspicion of criminal activity.”

She said random searches “can invite the possibility of racial, ethnic, or religious profiling.”

She said not only is the plan a bad one, but she predicted it would not make New Yorkers any safer.

Norman Siegel, the former head of the New York Civil Liberties Union who is now a lawyer and a candidate in the Democratic primary for public advocate, said that normally, police can search people only when “individualized suspicion” exists. But he said when there’s an “overriding interest,” the police can also search people randomly. For example, he said, police can conduct random traffic stops during holiday seasons to deter and catch drunk drivers. He cautioned, though, that in those cases, the police may not stop all black drivers, and in the case of the new subway searches, the police may not stop all people who appear to be Muslim.

“That would be unconstitutional,” he said. “The question becomes what is the criteria for stopping people.”

He added that it seems like stopping people who have bulky bags is different from stopping people at random.

“It’s an unfortunate sign of the times,” he said. “It’s a further erosion of people’s civil liberties. We will have to closely monitor how they implement their newly announced plan.”

New York subway riders interviewed yesterday had mixed feelings about the plan.

“I would have felt it was an invasion of privacy a month ago, but today, it might be a necessary measure,” a passenger on a downtown no. 6 train, Siri Peterson, said. “In light of what happened today, it makes me feel like at least they’re doing something.”

Michael Schultz, who was riding an F train yesterday afternoon, said, “Logistically, it sounds like a nightmare and a waste of time.” He said, however, that he understood why it was being done. “It’s an invasion of privacy, but if you’re not carrying anything illegal, you’ve got nothing to hide.”

Another rider, Yaw Mansuoh, was less accepting: “They shouldn’t search people’s bags. They should just watch and listen. When I go to catch a train and I’m late and you’re going to search my bag? No good. They should be a presence, that’s it.”

While the mayor and the police commissioner acknowledged that the procedure might make getting around the city slower for some people, they said speedy travel wasn’t as important as security.

“More importantly than avoiding any kind of slowdown is to make sure that people get where they need to go safely,” Mr. Bloomberg said. “We just live in a world where, sadly, these kinds of security measures are necessary. Are they intrusive? Yes. A little bit.”

The subway searches will focus on passengers boarding trains bound for Manhattan in the morning, Mr. Browne said. During the evening rush, the police will focus on trains exiting Manhattan.