Acknowledging that the city’s transit systems are not impervious to attack, officials from the New York Police Department said police would shut down all modes of transportation should one of those systems face an incursion. Those attacks, law enforcement officials suggested, could be at the hands of Islamic extremists who reside in New York City and maintain an ideology similar to that of the men accused of bombing London last month.

In assessing the city’s vulnerabilities, the Police Department’s anti-terrorism experts carefully outlined parallels between last month’s two series of attacks in the British capital but stopped short of saying they were directly connected. The comments came in a briefing with about 300 directors of security from private companies, as part of the new NYPD Shield program.

The deputy commissioner of counterterrorism, Michael Sheehan, said a definitive connection had not been made between the successful bombings on the morning of July 7 and the failed bombings at lunchtime 14 days later. He said authorities think the suspects were suicide bombers.

In the first attacks, the four suspects – three British citizens of Pakistani descent and one native Jamaican – drove two vehicles from Leeds to the Luton British Rail Station, where they boarded a train to King’s Cross Station in London. They split up and detonated three bomb devices simultaneously on Underground trains during the morning rush hour. Nearly an hour later, a fourth bomb exploded, blowing off the back of a double-decker bus. Other bombs were found later. In all, 56 people died and 700 were injured. Of the suspects, three of whom were born Muslims and one of whom converted at age 16, only one had a criminal history, Mr. Sheehan said.

Two weeks later, London experienced an attempted strike. On July 21, the suspects, four men of African descent, failed in their attempts to detonate four explosive devices in three Underground trains and one double-decker bus. Two days later, authorities found a fifth explosive device in shrubbery in the area. Only one person was injured and there were no fatalities. According to the deputy commissioner of intelligence, David Cohen, a suspect in custody said the terrorists had intended only to scare people.

On both dates, jihadists with international ties used bombs that were simply constructed and easy to assemble, the authorities said. The explosives were thought to be hexamethylene tri-peroxide or peroxide-based, and Mr. Cohen called them “homegrown.”

“This was not brain surgery,” Mr. Cohen said. The experts indicated it was easy to purchase the ingredients online or at a hardware store or a beauty-supply shop.

The NYPD Shield program is an initiative intended to allow police to disseminate terrorism-related information to the private sector in an industry-specific manner. Mayor Bloomberg and the police commissioner, Raymond Kelly, announced the plan July 26.

Mr. Kelly called the public-private partnership “a one-stop shopping initiative,” and participants include corporations such as IBM and Citicorp as well as nonprofit organizations such as Columbia University.

The director of security at Memorial Sloan-Kettering Cancer Center, James Baranello, said the terrorism related information was useful in keeping hospital administrators abreast of security notices in New York City and elsewhere.

The managing director of the Excel Security Corporation, Gerard Kane, said he wants to receive any available information about the London attacks.

“If you’re in charge of security, you have to be aware of what happened so you can be aware of what might happen here,” Mr. Kane, a former police sergeant, said. It is imperative, he noted, for security personnel to know how to detect suspicious packages and people. All of the security measures make it more difficult for terrorists to strike, he said. Inevitably, however, “something is going to happen,” Mr. Kane said. Excel oversees security in 300 Manhattan buildings.