Ms. Moyer-Sims, 20, said members of the Police Department’s intelligence division asked about her personal history, her relationship with other protesters, the nature of Occupy Wall Street and plans for upcoming protests.

“I felt like I had been arrested for a thought crime,” she said.

Mr. Pawar said that the police had charged his three clients, Ms. Moyer-Sims, Angela Richino and Matthew Vrvilo, with obstructing governmental administration, but that the Manhattan district attorney’s office had declined to prosecute them.

Now they are preparing to sue the city, Mr. Pawar said, adding that the arrests had violated their constitutional rights.

“Not only are the police disrupting people’s rights to free expression,” Mr. Pawar said. “They are taking pre-emptive steps by arresting people who might be just thinking about exercising their rights.”

Though Occupy Wall Street has largely faded from the headlines, organizers are planning springtime demonstrations in an effort to revitalize their movement. And they are troubled by what they consider continued monitoring by the police.

In 2003, citing the dangers of terrorism, a federal judge granted expanded surveillance powers to the New York police, who had previously faced restrictions in monitoring political groups. Police Commissioner Raymond W. Kelly, Mayor Michael R. Bloomberg and others have said the new latitude is essential to keeping the city safe.

But the Police Department’s surveillance efforts have recently gained attention and criticism with reports that officers compiled detailed data on Muslim communities. Now, some Occupy protesters worry that they are being subjected to similar scrutiny.

For the last few months, protest organizers say, police officers or detectives have been posted outside buildings where private meetings were taking place, have visited the homes of organizers and have questioned protesters arrested on minor charges.

“The N.Y.P.D. surveillance does not appear to be limited to unlawful activity,” said Donna Lieberman, the executive director of the New York Civil Liberties Union. “We count on the police, of course, to be on the lookout for terrorists and terrorism, but to think you could be on that continuum just by going to a peaceful protest is nuts.”

A police spokesman did not respond to a request for comment.

Undercover officers are generally entitled to attend public political gatherings. Protesters said apparent efforts to keep tabs on them had included officers’ showing up at private meetings and what some described as attempts at intimidation.

Ashley Cunningham, an Occupy organizer, said that on Dec. 16, officers parked outside her home in Bedford-Stuyvesant, Brooklyn, where people were discussing a demonstration planned for the next day.

Another organizer, Sandy Nurse, said she arrived at her apartment building in Bushwick, Brooklyn, on Dec. 16 and found uniformed officers outside who told her they were there to conduct a “security check” for a condition they would not identify.

Although she told them they could not enter, Ms. Nurse said, an officer used his foot to prevent the front door from closing behind her, followed her into the building’s entryway vestibule, and threatened to arrest her for obstruction of government administration. Ms. Nurse said the visit did not feel like a coincidence.

“It means that they are watching us,” she said. “They know who we are, where we live and where we are organizing.”

At various other points, organizers said, officers have posted themselves outside a building in the financial district where organizers were meeting, and stood in a hallway outside of an art studio in Dumbo, Brooklyn, where protesters were making signs and banners.

Several people charged with offenses like trespassing said detectives from the intelligence division questioned them, with other officers explaining that it was because of their connection to Occupy Wall Street.

Some of those questioned said the detectives seemed mainly interested in knowing about coming demonstrations. But sometimes, protesters and lawyers said, the questioning went further.

Mark Adams, a 32-year-old engineer from Virginia, said he was arrested in November at an Occupy Wall Street protest in Midtown and was questioned by a police detective and an agent from the Federal Bureau of Investigation, who asked about his involvement with Occupy Wall Street, requested his e-mail address and inquired whether he had ever been to Yemen or met anyone connected to Al Qaeda.

Mr. Adams, a naturalized United States citizen who was born in Pakistan, said he was arrested during another protest in January and questioned by intelligence division detectives. In that instance, he said, the detectives asked him about specific names and addresses, asked about his work history, education and family, and questioned him about a trip he had made to Ireland.

Mr. Adams said he was disturbed that anyone would consider him a threat because of his ethnicity or political views. “It’s scary,” he said.