A federal appeals court has dealt a blow to a civil-liberties lawsuit against the New York Police Department, saying the police force is within its rights to keep secret some 1,800 pages of documents about its surveillance of protesters ahead of the Republican National Convention in 2004.

More than 1,800 people were arrested at or near Madison Square Garden over four days in August, 2004, during the convention where President George W. Bush accepted his party’s nomination for a second term.

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According to court records, the NYPD sent undercover officers around the world ahead of the convention to collect information on people who planned to protest at the event.

The New York Civil Liberties Union filed a lawsuit against the NYPD on behalf of protesters who were arrested, handcuffed and fingerprinted. In its attempt to get the police to release all documents related to the arrests, the NYCLU ran into a roadblock when it came to some 1,800 pages of documents chronicling the NYPD’s surveillance efforts.

On Wednesday, a panel from the US District Court of Appeals for the Second Circuit ruled (PDF) that the NYPD can keep the documents secret, because releasing them would jeopardize police investigations by releasing the identities of the NYPD’s undercover officers.

The New York Times reports that the case “represents the largest legal challenge to the powers extended to the police since the Sept. 11 attack.”

The NYT‘s City Room blog reports that the NYCLU had wanted the documents to challenge the NYPD’s assertion that the surveillance gave the police enough justification for the mass arrests.

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But Chris Dunn, a lawyer for the NYCLU, said his group still has enough material from the NYPD to press ahead with its lawsuit challenging the arrests.

“The police documents remaining in the case are the 600 pages already produced to us, and nothing in those reports begins to justify the NYPD’s mass arrest, prolonged detention, and blanket fingerprinting of law-abiding protesters and bystanders during the Convention,” Dunn told the Associated Press.

The Times reports that some protesters caught up in the mass arrests have already settled separate lawsuits with the city:

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In recent years, the city has paid millions of dollars to many of those who sued after their arrests, asserting that they had been improperly taken into custody. Many were herded into pens at a West Side pier in Manhattan that was dubbed GuantÃƒÂ¡namo on the Hudson.

The appeals court’s ruling overturns a federal judge’s order last year that the city hand over the documents to the NYCLU. That judge had upheld an earlier ruling that the documents could be released with sensitive details “redacted” so as to hide the identities of undercover officers.

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But the appeals court rejected that argument, saying that the lower court had made “errors” in its ruling. The plaintiffs, the appeals court ruled, hadn’t proved a “compelling need” to have the documents released.

The court has Ã¢â‚¬Å“wisely recognized the fundamental importance to public safety of protecting undercover identities and maintaining the integrity of their methods,Ã¢â‚¬Â NYPD Commissioner Raymond W. Kelly told the Times. “This was an important decision for New York and for the protection of society at large.”