In a mixed ruling, a Manhattan federal judge said yesterday a city policy restricting large gatherings on Central Park’s Great Lawn is constitutional but that a jury should decide whether the city acted fairly in 2004 when it denied permits to activists seeking to rally in protest of the Republican National Convention.

The case has pitted civil libertarians and anti-war picketers against the city and park advocates, delving into minutiae about which kinds of crowds cause irreparable grass damage.

Two of the plaintiffs, the National Council of Arab Americans and Act Now to Stop War and End Racism, have said the city acted capriciously and unconstitutionally when it denied their permit to rally on the famed 13-acre area of the park.

The rules, which were put into effect to protect the grass following a major restoration in the late 1990s, require people holding gatherings of larger than 20 people to apply for a special permit. The protesters who sued note the city permits large concerts on the Great Lawn by pop artist Dave Matthews band as well as annual performances by the New York Philharmonic and the Metropolitan Opera.

The judge, William Pauley III, said a jury would need to figure out whether the city treated the protesters unfairly.

“A trial is necessary to determine whether the decision to deny plaintiff’s permit request violated their rights under the First Amendment,” the judge wrote.

A deputy chief at city’s Law Department, Robin Binder, said she is confident a court will vindicate the city’s 2004 decision. “We are gratified that the court upheld the Parks Department’s ability to limit events on the Great Lawn to those that will not cause damage to the Great Lawn and other lawns and landscapes in the City’s park system,” she said in a statement.

A message left last night with the Arab council wasn’t returned, and the voice mailbox at ANSWER in Washington, D.C., was full and not accepting new messages.