WASHINGTON - As thousands of New Yorkers began submitting to searches of their backpacks and briefcases before riding the nation's largest subway system, a civil liberties group said it was planning a possible lawsuit.

But legal scholars predicted the city's new policy would likely survive a court challenge.

The New York Civil Liberties Union asked subway riders to report their experiences with searches by completing an online form and said it would decide by Friday whether to sue.

Hours after last week's attempted terror bombings on the London Underground, New York police began inspecting bags carried by subway passengers.

A police spokesman said the searches would be based on numeric sampling, to make them random, and would be conducted before passengers entered the turnstiles, giving riders the option of walking away and not boarding the subway if they do not wish to have their possessions searched.

New York Mayor Michael Bloomberg urged riders to be accepting of the new security measures, saying, "It may take you a little longer to get where you're going, but we're going to make sure that you get there safely."

Civil libertarians cry foul
Some civil libertarians have already concluded the search policy violates the 4th Amendment's ban on "unreasonable searches and seizures."

"It represents a change in the balance between citizen rights to privacy and the ability of the state to intrude on that," said Bill Goodman of the Center for Constitutional Rights.

Such an intrusion might be acceptable if it proved effective, Goodman said, but he argued the searches were a "totally ineffective measure."

"People who don't want to be searched will simply turn around, walk away, go to the next subway stop, and keep doing that until they get in," he said.

Even so, experts on search and seizure law predicted the policy would be upheld if challenged in court.

Racial profiling?
"There's a sort of implied consent," said Thomas Davies of the University of Tennessee College of Law.

Searching subway passengers would be similar to searching airline passengers or even spectators at major sporting events, Davies said.

"If you don't want to be searched, you don't have to go to the football game or take the subway," he said.

Courts would also look at whether the subway searches involved any kind of racial profiling or other form of unconstitutional discrimination.

"They have to have procedures for who gets selected, whether it’s every fifth or tenth person, which involve neutral criteria," said David Aarsonson of American University's Washington College of Law.

"Can they focus on Arab Muslim men? Probably not," he said.

Davies said courts have found randomness to be an acceptable method of avoiding discrimination.

"It's easier to defend a random search than it would be one where the officer just looks for people that they have a hunch about, but not reasonable suspicion," Davies said.

‘Special need’
The city would also likely argue that its subway searches meet a "special need" of preventing terrorism.

While federal courts generally require that police demonstrate some kind of suspicion in order to perform a search, some exceptions have been made for searches that meet a specific need of the government.

Among examples allowed by the courts: random testing of student athletes or participants in extracurricular activities to deter drug use, drug and alcohol tests for railway employees to encourage safety, car stops at borders to intercept illegal immigrants, and sobriety checkpoints to remove drunken drivers from the roads.

But Michael Seidman of Georgetown University's School of Law believes the New York subway searches will be upheld for yet another reason.

"The federal courts invented the 'special needs' doctrine, and now they'll invent some more doctrine," to allow New York's approach.

"The way we balance law enforcement and privacy interests is contingent on the situation at the time they're balanced. People feel differently about it now than they did 10 years ago," Seidman said.