Mayor Bloomberg and the police commissioner, Raymond Kelly, sent thousands of extra police officers into New York City’s subways and to other vulnerable locations in the wake of last week’s terrorist bombings in London’s Underground.

No terrorism-related arrests have been made in New York since the city deployed at least one police officer to each train running beneath the streets, but the number of major crimes in the subways in the week after Independence Day plunged to 44 this year from 77 last year, according to statistics provided yesterday to The New York Sun.

Between July 4 and July 10 last year, 20 robberies, 50 grand larcenies, six felony assaults, and one rape were recorded in the subway system. In the same period this year, 15 robberies, 27 grand larcenies, two felony assaults, and no rapes were recorded, the police data showed.

The London bombings occurred early last Thursday, July 7.

The plunge in overall subway crime can’t be explained away by overall trends. It came during a year when major crimes in the subway system have risen 3%, and it was not accompanied by a jump in street-level crime – indeed, citywide crime in the July 4-10 period fell by 5%, the same as the year-to-date drop.

“I’m not surprised. Most crimes are crimes of opportunity,” a senior lawyer for the Straphangers Campaign, Gene Russianoff, said. “A uniformed presence makes for a strong deterrent in crime.”

Mr. Russianoff said the success of the past week raises an important question: Will the number of officers assigned to the subway system drop back to its original level once the terrorist threat diminishes, or will it stay at its present level? He said he would like to see the number of officers patrolling the subways remain higher than it was before the London attacks.

The member of the New York City Council who heads the Transportation Committee, John Liu, said: “Reducing crime in the subways, or anywhere else for that matter, is a good thing. “He said the lesson of the one-week swell in officers is “More cops, less crime.”

Mr. Liu, a Democrat of Flushing, said the city should maintain a strong police presence in the subway system, but that it might be impossible because adding police officers comes at a high cost to city taxpayers.

Asked whether the police presence would remain high, Mr. Bloomberg told reporters yesterday:

“You’re not going to see police officers in every train, on every station platform, on every bus, and every once in a while when you least expect it, you’ll see a massive display of force.

“We won’t ever know whether that person sitting next to you is an undercover police officer,” he continued, “and that’s exactly the way it should be.”

He added: “No city can afford to put a police officer next to every individual, nor do we want it because it’s such an imposition and an infringement on our rights.”

During the evening rush hour yesterday, subway riders expressed mixed feelings about the impact of a visible police presence on their trains.

“It’s a deterrent to basic, everyday subway crime,” an employee of an investment bank, Mari Dubois, said. Ms. Dubois said when she moved to New York City from Texas 12 years ago, she saw a robber rip a necklace from a woman’s neck while riding a subway on a Friday afternoon. Despite that, Ms. Dubois said, she has never felt unsafe on the city’s subways and buses – but she feels safer when there are more officers patrolling.

“I love having them here,” she said.

Another subway rider, Elle Skeath, who works at a bar in Lower Manhattan, said that after she read about a recent rape in a subway station, she was nervous about riding the trains at night. She said that since she started noticing more officers on the subways, she has felt more confident.

“It makes you more secure. You don’t feel as nervous,” she said.

A commuter, Matthias Cheng, said the police should definitely continue to monitor the stations and the trains, but he said the city probably doesn’t need to keep one officer on each train, as it has since morning rush hour last Thursday.

“At the end of the day, the taxpayers pay the bill,” Mr. Cheng, who works in information technology, said.

A public-school teacher, Aaron Orzech, said he has noticed that police officers have not only been more plentiful in the last week, but they have also been more alert. He said, however, that he was skeptical that the police officers could stop terrorists.

“I feel that they think they have to do it – the city does – but I don’t think it’s going to make a difference,” he said. “They can’t keep doing this forever. The terrorists are going to strike when we don’t expect it.”

A sophomore at John Jay College majoring in criminal justice, Sandy Mun, also questioned whether the officers could actually prevent a terrorist attack.

“I think it’s really pointless,” she said. “God forbid something happens – what could they do?”

Since Thursday, the police have not reported noticing any terrorist-related activity aboard the subway cars, although officers did bring in for questioning some people who were videotaping in a subway in Brooklyn. The taping turned out to be innocuous, a Police Department spokesman, Jason Post, said.

Mr. Post said the police commissioner would keep the extra officers in the system at least through rush hour this Friday afternoon. On Friday, Mr. Kelly will reassess the heightened police presence, the spokesman said.