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.