Responding to the terrorist train bombings that killed more than 150 in Mumbai, India, the New York City Police Department sharply increased security in the city’s subway system yesterday, officials said.

Mayor Bloomberg and the police commissioner, Raymond Kelly, were quick to point out that there was no indication that terrorists planned a similar attack in New York. Precaution, they said, is the city’s best response when terror plots are carried out around the world. Is “an event part of a worldwide operation and plot?” Mr. Kelly asked. “We don’t know.”

Though the Department of Homeland Security did not raise the nation’s threat level yesterday, an unrelated subway derailing in Chicago in the evening added to the country’s sense of alarm. Dozens of people were treated for smoke inhalation and subway service was stopped on Chicago’s Blue Line for hours. An early investigation showed no signs of terrorism, officials said.

In New York, police held their second platoon through the evening rush hour. The platoon usually starts duty at 7 a.m. and finishes after 3:30 p.m. Random bag checks were increased and a special bomb detection unit was inspecting some subway stations, police said.

A daily counterterrorism maneuver called the Critical Response Vehicle “surge,” where 125 or so police officers in more than 76 squad cars flood areas of the city, focused on the subway infrastructure, the police department’s chief spokesman, Paul Browne, said. Officers at some subway stations performed a maneuver called “Total Order Maintenance,” where officers enter every train car on a subway station simultaneously, Mr. Browne said.

Mr. Kelly said the department also was considering sending one of the NYPD detectives stationed abroad, possibly the one in Singapore, to India. As of last night, a final decision had not been made, Mr. Browne said. With funding from the Police Foundation, detectives are stationed in nine countries, including Britain, Israel, and Australia.

When coordinated bombs exploded in the London public transportation system on July 7, 2005, the London-based NYPD detective immediately phoned in a firsthand account to One Police Plaza, prompting Mr. Kelly to hold over the day platoon and create the department’s policy of random bag searches.

The Metropolitan Transportation Authority also beefed up the number of personnel in the system yesterday, including ferries, bridges, and tunnels, a spokesman,Tim O’Brien, said.

Mr. Bloomberg urged New Yorkers not to be alarmed by the heightened security presence in the city.

“As we all know, vigilance is just the reality of the post-9/11 world, and I would urge New Yorkers not to worry, trust that the professionals are on top of it, and continue going about life as normal,” he said.

He emphasized that some of the security measures being taken by the city would not be made public, in order to keep potential terrorists in the dark about what they are up against.

“The person sitting next to you on the subway — who is probably the most unlikely looking police officer in the world — is very likely to be a police officer,” Mr. Bloomberg said.

In the last year, the security of public transportation systems has come under increased scrutiny. With the devastation caused by the Madrid railway bombings, London subway and bus bombings, and now the Mumbai commuter rail bombings, some observers have said it is only a matter of time before an attack is attempted against New York City’s vast transportation system of 468 stations, which serves 4.5 million riders daily. In the last month, two plots against city subways have been uncovered, including one in which terrorists would explode bombs on PATH trains as they traveled under the Hudson River.

Since the attacks of September 11, 2001, the MTA police have increased their force by 40% and have added an Emergency Services Unit. The authority is in the process of adding 15 dogs to the 35-strong K–9 unit. This spring the authority began hardening the subways, including the underwater tunnels, to make them more resistant to explosions.

The MTA also has a $212 million contract with Lockheed Martin to install several thousand cameras and sensors across the system over the next 2 1/2 years. The program’s timeline has drawn criticism from City Council members, who said in recent hearings that the subways are still wide open to an attack nearly five years after September 11.

A former MTA chairman and the London transport’s current commissioner, Robert Kiley, cast doubt on the city’s response to the London bombings in early May, according to an article in Newsday.

“I don’t believe there’s a technological solution to keep this from happening again,” he said, according to the article. “Rapid transit is the most vulnerable piece of infrastructure you could possibly imagine.”