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(The following story by Sewell Chan and Kareem Fahim appeared on the New York Times website on July 22.)

NEW YORK — The police last night began random searches of backpacks and packages brought into the New York City subways as officials expressed alarm about the latest bomb incidents in the London transit system.

The searches, which will also include commuter rail lines, are not a response to a specific threat against the city, said Mayor Michael R. Bloomberg, who authorized the searches shortly before he announced them at a morning news conference.

The police have previously inspected bags at major events like parades and demonstrations, and the authorities in Boston conducted random baggage searches on commuter rail lines during the Democratic National Convention last year, but officials here could not recall a precedent for a broad, systematic search of packages in the New York City subways, which provide 4.7 million rides each weekday.

At some of the busiest of the city’s 468 stations, riders will be asked to open their bags for a visual check before they go through the turnstiles. Those who refuse will not be permitted to bring the package into the subway but will be able to leave the station without further questioning, officials said.

Police Commissioner Raymond W. Kelly promised “a systematized approach” in the searches and said the basis for selecting riders for the checks would not be race, ethnicity or religion. The New York Civil Liberties Union questioned the legality of the searches, however, and Mr. Kelly said department lawyers were researching the constitutional implications.

“Every certain number of people will be checked,” Mr. Kelly said. “We’ll give some very specific and detailed instructions to our officers as to how to do this in accordance with the law and the Constitution.”

Paul J. Browne, a Police Department spokesman, said officers would focus on backpacks and containers that are large enough to carry explosive devices or ordnance. “We have some history of what those look like,” he said. “They’re bigger than a handbag.” Officers are unlikely to search pocketbooks, he said.

Searches began last night at several stations, including 14th Street-Union Square in Manhattan and an undisclosed station along the No. 7 line near Shea Stadium, in Queens. Today, the first full day the searches will be conducted, two of the many stations to be checked are Woodlawn-Jerome Avenue, on the No. 4 line in the Bronx, and Lafayette Avenue on the C line in Brooklyn. Mr. Browne said the search policy would continue indefinitely.

Transit officials in several other cities – Boston, Washington and San Francisco – said they were considering similar measures, although few have actually started randomly checking bags. A spokesman for the Bay Area Rapid Transit in San Francisco said officials were not certain whether they have the legal authority for such searches. “This could be the lawyer’s dream case,” said the spokesman, Linton Johnson. “There is this balance of civil liberties and protection.”

Lisa Farbstein, a spokeswoman for the Washington Metropolitan Area Transit Authority, which carries 1.2 million subway and bus passengers each weekday, said officials in the capital would watch how the effort went in New York. “It could be an option for us,” she said, “but we are not there yet in terms of an implementation plan.”

After the July 7 explosions in London, transit officials in Atlanta and Salt Lake City notified passengers that they reserved the right to inspect packages and bags, but the number of searches has been very small. In Utah, where a 20-mile rail system carries 45,000 passengers a day, a total of two bags have been inspected.

In Boston, for two weeks before the Democratic convention, subway stations were selected at random and bags were checked before riders entered the system, said John Martino, deputy police chief at the Massachusetts Bay Transportation Authority. Police ran swabs across the bags and then put the swabs in machines that could detect explosives. “When we did it, we actually had people asking to be screened,” Chief Martino said yesterday in a telephone interview. “It makes them more comfortable knowing that it was being done.”

William W. Millar, president of the American Public Transportation Association, an industry group, said comprehensive coverage of any major urban transit system would be next to impossible. “If you were going to try to check a very high percentage at every station or on every train, it would be incredibly labor-intensive,” he said.

Still, he said, the searches could deter would-be attackers and improve the public’s confidence. “The public wants to feel safe, as well as be safe,” he said. “So this has a benefit of perception.”

Mr. Kelly said his department would “reserve the right” to expand the searches to buses and ferries, and he made it clear that many subway riders will be affected. “Ideally, it will be before you go through the turnstile,” he said. “You have a right to turn around and leave, but we also reserve the right to do those types of searches if someone is already inside the system.”

At the selected stations, as many as one in five or one in ten passengers may be picked for a search, said Mr. Browne. Supervisors will check that the searches are being randomly conducted, he said.

The Metropolitan Transportation Authority said its own smaller police force would conduct similar searches on the Long Island Rail Road and Metro-North Railroad. At Grand Central Terminal, an announcement was repeated over the loudspeakers last night: “Passengers are advised that their backpacks and other large containers are subject to random search by the police.”

Mr. Bloomberg acknowledged that passengers might be inconvenienced. “It’s a complex world where, sadly, there are a lot of bad people,” he said. “We know that our freedoms are threatening to certain individuals, and there’s no reason for us to let our guard down.”

The mayor said he spoke with Gov. George E. Pataki and with the secretary of homeland security, Michael Chertoff, shortly after hearing about the attacks in London yesterday, two weeks to the day after four bombings in the transit system there killed 56 and injured 700.

The police will focus on stations with heavy Manhattan-bound traffic in the morning and on stations with commuters leaving Manhattan in the evening. Riders will be asked to open their bags or allow them to be sniffed by trained dogs.

Mr. Browne, the police spokesman, said, “Obviously we’re going to use common sense for someone that appears to be an imminent threat.” For example, he said, if a passenger with a large package had both fists clenched, police officers would be justified in searching him. Anyone found to be holding illegal drugs or weapons is subject to arrest, he said.

The Transit Bureau of the Police Department has 2,200 officers and 500 supervisors, and even with the hundreds more that have been added for subway patrols, it is unclear how many riders can feasibly be searched. At Times Square, for example, there are 165,876 turnstile clicks on a typical weekday. Some of the system’s turnstiles are used by a dozen passengers a minute.

Mr. Browne said such searches had been discussed “from time to time, over the last three years.” Mr. Kelly suggested that riders could voluntarily speed the process. “Ideally, people wouldn’t carry any backpacks or bulky packages on the transit system,” he said.

Some riders expressed cautious support. Hani Judeh, 24, a Palestinian-American medical student who lives in Brooklyn, said he shaved his beard, stopped speaking Arabic publicly and attended mosque less regularly after 9/11.

He said he favored the searches, as long as they did not involve racial profiling. “They should check bags, but they can’t discriminate,” he said. “You can’t tell Indian from Pakistani, you can’t tell West Indian from black, you can’t tell Arab from Mediterranean.”

(Reporting for this article was contributed by Jennifer 8. Lee, Eric Lipton, Patrick McGeehan and Shadi Rahimi.)