Defending the decision to conduct random searches of subway passengers, one of the Police Department’s highest-ranking counterterrorism officials told a federal judge yesterday that “in a perfect world,” police would inspect bags at all of the system’s 468 stations.

“More is better than some, some is better than none, and none helps” the terrorists, the deputy commissioner of intelligence, David Cohen, said.

Mr. Cohen, who came to the Police Department in 2002 after spending more than 30 years at the Central Intelligence Agency, told Judge Richard Berman in U.S. District Court in Manhattan that the randomness of the program created enough uncertainty to disrupt the execution and planning of potential terror attacks.

The case brought against the city by the New York Civil Liberties Union en ters its second day today, when examination of witnesses is likely to conclude. The judge is expected to rule sometime in December.

The NYCLU argued yesterday that the police program is enforced too sporadically to be effective and, as a result, did not justify the suspension of the Fourth Amendment of the Constitution, which protects against random searches.

“It’s difficult to believe sophisticated terrorists who are trying to attack the subway system are going to be deterred,” the NYCLU’s lead attorney, Christopher Dunn, said during his opening statement. “The only thing being searched are innocent New Yorkers.”

Mr. Dunn told Judge Berman yesterday that ruling the searches constitutional would mark “a momentous change in our society.”

The city’s random bag search policy, the first of its kind in America, began July 22, a day after a second bombing attack on the London transit system. News of the second attack prompted Police Department lawyers to draft the policy, which they then immediately implemented, the deputy director of the Police Department’s law office, Kerry Sweet, told the court yesterday.

The five plaintiffs in the case against the city faced cross-examination yesterday. They were asked to explain why, for the most part, they do not object to the searches that have become commonplace at private and public places like Madison Square Garden, Shea Stadium, and the city’s airports and courts.

In fact, lawyers for the city pointed out that a sign posted at the office building at 124 Broad St., where the New York Civil Liberties Union has its headquarters, tells visitors of the building’s right to search bags.

One plaintiff, Joseph Gehring Jr., the son of a retired police captain, said that because the subway is a necessity of daily life for most New Yorkers, it is an extension of the public sidewalk and should not be considered in the same light as a self-enclosed space like a sports arena, courthouse, or airport.

Mr. Gehring, who is a lawyer, said the Police Department’s subway search methods make him question whether he could be searched anywhere in the city.

“I don’t know what my rights are, at least as far as the city sees my rights,” Mr. Gehring said. He added that he “does not want the searches to justify other searches.”

The other plaintiffs, all of whom made similar arguments, include a survivor of the September 11, 2001, attacks, Brendan MacWade; a social worker, Andrew Schonebaum; an employee of the federal government, Norman Murphy, and a writer and political activist, Partha Banerjee.

Lawyers for the city tried to discredit a survey conducted by the NYCLU between August 25 and September 16 that the group said shows that searches are not frequent enough to offer meaningful value as a deterrent and therefore warrant the suspension of the Fourth Amendment.

The NYCLU said that during the 23-day period, 13 “observers” who were paid $10 an hour canvassed the system’s 468 stations and recorded 34 instances when they witnessed police searches taking place.

Judge Berman asked whether anyone considered using sampling techniques favored by academics in polling and surveys. A witness said Mr. Dunn had designed the methodology, an answer that did not appear to satisfy the judge.

The police have divulged little detail about the search program and say they do not keep any statistics about the searches. The police did offer some insights about the program, saying the frequency of inspections are increased during rush hour, when an attack could inflict more damage. Also, the searches have yet to lead to an arrest, the deputy chief of the police transit bureau, Owen Monaghan, said.

The civil liberties union chose not to cross-examine Mr. Cohen and another witness for the defense, the deputy commissioner of counterterrorism, Michael Sheehan.

After Mr. Cohen was excused as a witness, opponents of the search policy loudly criticized the judge, who alone had questioned the witness, for not asking tougher questions.

“This is not interactive TV,” Judge Berman said. The two protesters stood up to leave.

“It’s nonsense. It’s sophistry,” they shouted.

While testimony in the case is likely to conclude today, final arguments will not be heard until December 2, after which the judge will rule on the case.