Police said they arrested a suspect who was captured on videotape stomping a detective Monday night near Madison Square Garden, and officers want to interview a legal observer who witnessed the assault.

However, the National Lawyers Guild observer has taken her case to an attorney, who has accused the police of provoking the assault with their aggressive tactic of driving scooters into a crowd of protesters while wearing plainclothes.

“It looked to her like this was madness, like someone rode a motorcycle into people and started hitting people,” said attorney Daniel Alterman, who would not name his client “because of the potential detriment to her career, her well-being, and her status.”

Mr. Alterman’s client, whom he described as a 30-something computer specialist, appeared in a videotape of the assault that was widely distributed by the press and police. In the clip, she is wearing the distinctive neon green cap of those appointed by the National Lawyers Guild to observe, and occasionally videotape, police activity during protest marches.

Police Commissioner Raymond Kelly, in a press conference Tuesday, described the attack on Detective William Sample Jr. as “vicious,” particularly since he was coming to the aid of other officers. Mr. Kelly also said police have been “crowded” by guild observers over the last few days.

In response to allegations of rough police behavior, police officials have said that officers were responding to a violent crowd and an officer’s request for assistance. The New York Sun was unable to obtain a police response to the lawyer’s statements yesterday.

Police arrested Jamal Holiday, 20, of 231 E. 116th St., a suspect accused of assaulting and seriously injuring a plainclothes detective at about 8 p.m. Monday at the intersection of 29th Street and Eighth Avenue, as a march organized by The Economic Human Rights Campaign came to a close.

Mr. Holiday was captured on videotape punching and stomping Detective Sample as the detective lay prone on the street. Detective Sample was knocked off his scooter and he and several other plain-clothes police officers attempted to disperse a crowd by bumping pedestrians with their unmarked scooters.

Detective Sample suffered serious head injuries and was treated at St. Vincent’s Hospital, where he was released yesterday. Mr. Holiday, who was 19 at the time of the assault, was charged with second-degree assault and spent his 20th birthday in jail yesterday.

An independent filmmaker captured footage of the assault, featuring clear images of Mr. Holiday and the observer which were widely distributed by the press and the police. At 7:58 p.m. on Tuesday, two detectives from the Special Frauds Unit spotted Mr. Holiday participating in a protest at Union Square. Mr. Holiday, who was reportedly wearing the same clothes that he was wearing in the video, was arrested.