In a ruling certain to have an impact on next month’s Republican National Convention, a Manhattan federal judge ruled yesterday police cannot “pen” protesters without giving them some freedom to move and cannot search protesters’ bags without proving they could pose a terrorist threat.

The executive director of the New York Civil Liberties Union, Donna Lieberman, hailed Judge Robert Sweet’s ruling as a “victory” and “an important means of protection for First Amendment rights.”

The NYPD’s deputy commissioner of public information, Paul Browne, said the ruling has no impact on existing police procedures.

“The court endorsed policies that Police Commissioner [Raymond] Kelly had already put into practice,” Mr. Browne said.

“The decision does not cause the police department to change any plans or tactics for safeguarding the Republican National Convention or the demonstrations associated with it. In effect, the court accepted existing department practices as reasonable and rational,” he added.

Ms. Lieberman was happy nonetheless. “This is the first time that the court has acknowledged and confirmed the concerns that advocates have been voicing for years about the pens and that’s an important breakthrough for free speech,” she said.

“Judge Sweet has struck a reasonable balance between the First Amendment and concerns about safety, and we believe his ruling will lead to better and safer demonstrations in New York City,” she added.

Where metal barricades are used to enclose crowds, Judge Sweet said police must allow protesters to enter and leave the enclosed areas as they wish, instead of sealing off areas to new arrivals once they fill up, even after people leave and crowds in the pens begin to diminish. The ruling would allow protesters to return to barricaded areas after they leave to use restrooms or to get water.

Judge Sweet also said police could no longer conduct generalized bag searches because the city has “not shown that the invasion of personal privacy entailed by the bag search policy is justified by the general invocation of terrorist threats, without showing how searches will reduce the threat.”

But the judge said bag searches “may be justifiable under different circumstances” and that conducting searches with metal-detecting wands is still allowed.

Gail Donoghue, special counsel in the city law department, said the ruling has little practical effect because the city had already directed the police department to stop general bag searches when it concluded they might not be legal.

She said other aspects of the ruling do nothing to limit the city’s authority to safely monitor police demonstrations by whatever means it considers reasonable and appropriate.

“He has not hamstrung us,” Ms. Donoghue said of the judge’s ruling.

The ruling came after the New York Civil Liberties Union argued on behalf of several protesters who complained about police actions during a February 2003 anti-war protest.

In response to the ruling, William Dobbs, press coordinator for United for Peace and Justice, said, “It’s ironic that the NYPD gets its hand slapped on a day when they’re refusing to meet with us.”

Leaders of United for Peace and Justice criticized Mayor Bloomberg and Mr. Kelly for breaking off negotiations regarding their permit to hold a 250,000-person rally at the West Side Highway on August 29, the day before the Republican convention begins at Madison Square Garden.

“It’s very difficult for us to say ‘yes’ before we know how some of the issues will be resolved,” said United for Peace and Justice director Leslie Cagan. “It’s like signing a contract with a lot of empty spaces in it, a lot of blanks in it, and the blanks will be filled out later.”