In glimpses and in glaring detail, the videotape images reveal the robust presence of disguised officers or others working with them at seven public gatherings since August 2004.

The officers hoist protest signs. They hold flowers with mourners. They ride in bicycle events. At the vigil for the cyclist, an officer in biking gear wore a button that said, "I am a shameless agitator." She also carried a camera and videotaped the roughly 15 people present.

Beyond collecting information, some of the undercover officers or their associates are seen on the tape having influence on events. At a demonstration last year during the Republican National Convention, the sham arrest of a man secretly working with the police led to a bruising confrontation between officers in riot gear and bystanders.

Until Sept. 11, the secret monitoring of events where people expressed their opinions was among the most tightly limited of police powers.

Provided with images from the tape, the Police Department's chief spokesman, Paul J. Browne, did not dispute that they showed officers at work but said that disguised officers had always attended such gatherings - not to investigate political activities but to keep order and protect free speech. Activists, however, say that police officers masquerading as protesters and bicycle riders distort their messages and provoke trouble.

The pictures of the undercover officers were culled from an unofficial archive of civilian and police videotapes by Eileen Clancy, a forensic video analyst who is critical of the tactics. She gave the tapes to The New York Times. Based on what the individuals said, the equipment they carried and their almost immediate release after they had been arrested amid protesters or bicycle riders, The Times concluded that at least 10 officers were incognito at the events.

After the 2001 terrorist attacks, officials at all levels of government considered major changes in various police powers. President Bush acknowledged last Saturday that he has secretly permitted the National Security Agency to eavesdrop without a warrant on international telephone calls and e-mail messages in terror investigations.

In New York, the administration of Mayor Michael R. Bloomberg persuaded a federal judge in 2003 to enlarge the Police Department's authority to conduct investigations of political, social and religious groups. "We live in a more dangerous, constantly changing world," Police Commissioner Raymond W. Kelly said.

Before then, very few political organizations or activities were secretly investigated by the Police Department, the result of a 1971 class-action lawsuit that charged the city with abuses in surveillance during the 1960's. Now the standard for opening inquiries into political activity has been relaxed, full authority to begin surveillance has been restored to the police and federal courts no longer require a special panel to oversee the tactics.

Mr. Browne, the police spokesman, said the department did not increase its surveillance of political groups when the restrictions were eased. The powers obtained after Sept. 11 have been used exclusively "to investigate and thwart terrorists," Mr. Browne said. He would not answer specific questions about the disguised officers or describe any limits the department placed on surveillance at public events.

Jethro M. Eisenstein, one of the lawyers who brought the lawsuit 34 years ago, said: "This is a level-headed Police Department, led by a level-headed police commissioner. What in the world are they doing?"

For nearly four decades, civil liberty advocates and police officials have fought over the kinds of procedures needed to avoid excessive intrusion on people expressing their views, to provide accountability in secret police operations and to assure public safety for a city that has been the leading American target of terrorists.

To date, officials say no one has complained of personal damage from the information collected over recent months, but participants in the protests, rallies and other gatherings say the police have been a disruptive presence.

Ryan Kuonen, 32, who took part in a "ride of silence" in memory of a dead cyclist, said that two undercover officers - one with a camera - subverted the event. "They were just in your face," she said. "It made what was a really solemn event into something that seemed wrong. It made you feel like you were a criminal. It was grotesque."

Ms. Clancy, a founder of I-Witness Video, a project that collected hundreds of videotapes during the Republican National Convention that were used in the successful defense of people arrested that week, has assembled videotape of other public events made by legal observers, activists, bystanders and police officers.

She presented examples in October at a conference of defense lawyers. "What has to go on is an informed discussion of policing tactics at public demonstrations, and these images offer a window into the issues and allow the public to make up their own mind," Ms. Clancy said. "How is it possible for police to be accountable when they infiltrate events and dress in the garb of protesters?"

The videotapes that most clearly disclosed the presence of the disguised officers began in August 2004. What happened before that is unclear.

Among the events that have drawn surveillance is a monthly bicycle ride called Critical Mass. The Critical Mass rides, which have no acknowledged leadership, take place in many cities around the world on the last Friday of the month, with bicycle riders rolling through the streets to promote bicycle transportation. Relations between the riders and the police soured last year after thousands of cyclists flooded the streets on the Friday before the Republican National Convention. Officials say the rides cause havoc because the participants refuse to obtain a permit. The riders say they can use public streets without permission from the government.

In a tape made at the April 29 Critical Mass ride, a man in a football jersey is seen riding along West 19th Street with a group of bicycle riders to a police blockade at 10th Avenue. As the police begin to handcuff the bicyclists, the man in the jersey drops to one knee. He tells a uniformed officer, "I'm on the job." The officer in uniform calls to a colleague, "Louie - he's under." A second officer arrives and leads the man in the jersey - hands clasped behind his back - one block away, where the man gets back on his bicycle and rides off.

That videotape was made by a police officer and was recently turned over by prosecutors to Gideon Oliver, a lawyer representing bicycle riders arrested that night.

Another arrest that appeared to be a sham changed the dynamics of a demonstration. On Aug. 30, 2004, during the Republican National Convention, a man with vivid blond hair was filmed as he stood on 23rd Street, holding a sign at a march of homeless and poor people. A police lieutenant suddenly moved to arrest him. Onlookers protested, shouting, "Let him go." In response, police officers in helmets and with batons pushed against the crowd, and at least two other people were arrested.

The videotape shows the blond-haired man speaking calmly with the lieutenant. When the lieutenant unzipped the man's backpack, a two-way radio could be seen. Then the man was briskly escorted away, unlike others who were put on the ground, plastic restraints around their wrists. And while the blond-haired man kept his hands clasped behind his back, the tape shows that he was not handcuffed or restrained.

The same man was videotaped a day earlier, observing the actress Rosario Dawson as she and others were arrested on 35th Street and Eighth Avenue as they filmed "This Revolution," a movie that used actual street demonstrations as a backdrop. At one point, the blond-haired man seemed to try to rile bystanders.

After Ms. Dawson and another actress were placed into a police van, the blond-haired man can be seen peering in the window. According to Charles Maol, who was working on the film, the blond-haired man is the source of a voice that is heard calling: "Hey, that's my brother in there. What do you got my brother in there for?"

After Mr. Browne was sent photographs of the people involved in the convention incidents and the bicycle arrests, he said, "I am not commenting on descriptions of purported or imagined officers."

The federal courts have long held that undercover officers can monitor political activities for a "legitimate law enforcement purpose." While the police routinely conduct undercover operations in plainly criminal circumstances - the illegal sale of weapons, for example - surveillance at political events is laden with ambiguity. To retain cover in those settings, officers might take part in public dialogue, debate and demonstration, at the risk of influencing others to alter opinions or behavior.

The authority of the police to conduct surveillance of First Amendment activities has been shaped over the years not only by the law but also by the politics of the moment and the perception of public safety needs.

In the 1971 class-action lawsuit, the city acknowledged that the Police Department had used infiltrators, undercover agents and fake news reporters to spy on yippies, civil rights advocates, antiwar activists, labor organizers and black power groups.

A former police chief said the department's intelligence files contained a million names of groups and individuals - more in just the New York files than were collected for the entire country in a now-discontinued program of domestic spying by the United States Army around the same time. In its legal filings, the city said any excesses were aberrational acts.

The case, known as Handschu for the lead plaintiff, was settled in 1985 when the city agreed to extraordinary new limits in the investigation of political organizations, among them the creation of an oversight panel that included a civilian appointed by the mayor. The police were required to have "specific information" that a crime was in the works before investigating such groups.

The Handschu settlement also limited the number of police officers who could take part in such investigations and restricted sharing information with other agencies.

Over the years, police officials made no secret of their belief that the city had surrendered too much power. Some community affairs officers were told they could not collect newspaper articles about political gatherings in their precincts, said John F. Timoney, a former first deputy commissioner who is now the chief of police in Miami.

The lawyers who brought the Handschu lawsuit say that such concerns were exaggerated to make limits on police behavior seem unreasonable. The city's concessions in the Handschu settlement, while similar to those enacted during that era in other states and by the federal government, surpassed the ordinary limits on police actions.

"It was to remedy what was a very egregious violation of people's First Amendment rights to free speech and assemble," said Jeremy Travis, the deputy police commissioner for legal affairs from 1990 to 1994.

At both the local and federal level, many of these reforms effectively discouraged many worthy investigations, Chief Timoney said. "The police departments screw up and we go to extremes to fix it," Chief Timoney said. "In going to extremes, we leave ourselves vulnerable."

Mr. Travis, who was on the Handschu oversight panel, said that intelligence officers understood they could collect information, provided they had good reason.

"A number of courts decided there should be some mechanism set up to make sure the police didn't overstep the boundary," said Mr. Travis, who is now the president of John Jay College of Criminal Justice. "It was complicated finding that boundary." The authority to determine the boundary would be handed back to the Police Department after the Sept. 11 attacks.

On Sept. 12, 2002, the deputy police commissioner for intelligence, David Cohen, wrote in an affidavit that the police should not be required to have a "specific indication" of a crime before investigating. "In the case of terrorism, to wait for an indication of crime before investigating is to wait far too long," he wrote.

Mr. Cohen also took strong exception to limits on police surveillance of public events.

In granting the city's request, Charles S. Haight, a federal judge in Manhattan, ruled that the dangers of terrorism were "perils sufficient to outweigh any First Amendment cost."

New guidelines say undercover agents may be used to investigate "information indicating the possibility of unlawful activity"- but also say that commanders should consider whether the tactics are "warranted in light of the seriousness of the crime."

Ms. Clancy said those guidelines offered no clear limits on intrusiveness at political or social events. Could police officers take part in pot-luck suppers of antiwar groups, buy drinks for activists? Could they offer political opinions for broadcast or publication while on duty but disguised as civilians?

Mr. Browne, the police spokesman, declined to answer those questions. Nor would he say how often - if ever - covert surveillance at public events has been approved by the deputy commissioner for intelligence, as the new guidelines require.