Civil rights advocates told a City Council committee yesterday that hundreds of protesters were arrested unlawfully, mistreated, and detained for as long as 72 hours during the Republican National Convention.

During a hearing of the committee on governmental operations, representatives from the New York Civil Liberties Union said 20% of the estimated 1,800 people arrested were swept up in netting without regard for whether they had broken the law.

“Hundreds of people were arrested who did nothing wrong,” said the executive director of the NYCLU, Donna Lieberman. “They were arrested because they were near a demonstration, they were arrested because they were taking pictures of a demonstration, observing a demonstration, or participating in a lawful demonstration.”

Civil Rights lawyer Normal Siegel testified that one of his clients was walking out of the Second Avenue Deli with “matzo-ball soup, pastrami, corned beef sandwiches, and potato latkes” when he was arrested.

The hearing was the first since the mass arrests during the convention two weeks ago. Protesters have, however, been publicly claiming that conditions at the temporary detention center, Pier 57, were deplorable. They are also claiming that many were held for longer than the 24-hour legal limit.

The Bloomberg administration was noticeably absent yesterday. As a result, many questions, including why the processing system was backlogged and why no fingerprinting machines were at the detention center, went unanswered.

Council Member Bill Perkins, who headed the hearing, said he was prepared to subpoena the administration to testify.

“There is no justifiable reason for them not to be here,” Mr. Perkins said, noting that the police commissioner and mayor had discussed the case on television and written newspaper editorials on the issue.

The mayor’s spokesman, Ed Skyler, said with court proceedings under way the administration could not testify.

“We can’t send people to testify about matters which are in litigation,” Mr. Skyler wrote in an e-mail. “We explained this to the council member last week, but he is more interested in grandstanding than accomplishing anything of substance or real value.”

The mayor has denied that conditions at Pier 57 were unsuitable as well as allegations that the city intentionally held protesters for longer than necessary to keep them off the streets until the Republicans left town.

Lawyers for the Legal Aid Society say the city and court system had prepared for waves of arrests during the convention. The deputy attorney for the criminal defense division at the society said dozens of people arrested on charges unrelated to demonstrations were processed in 10 to 12 hours, far faster than the protesters.