The city's Parks Department immediately rejected the request by the group, United for Peace and Justice, raising the possibility of 250,000 people marching up Seventh Avenue from a staging area south of 23rd Street toward the convention site with no set destination.

The group said it still planned to march past Madison Square Garden on Aug. 29 - the day before the convention begins - but said it could no longer agree to rally along the highway, in part because so many of its constituents had objected to it, threatening to dilute the strength of the assembly.

Organizers said they rejected the West Side site after failing to secure amenities from the city like shuttle buses and water, and learning that many of their member organizations and individuals simply would not attend.

"We believe there is still time to turn this thing around," Leslie Cagan, national coordinator for United for Peace and Justice, said at a late-morning news conference at the group's Midtown offices. Even given the short notice, "It would be very easy for us to communicate to people that we're going to Central Park," she continued. "When there is the political will to get something done, then the practical pieces fall into place."

But the Bloomberg administration said it would oppose any attempt to hold the rally in Central Park.

"The city has worked hard to accommodate a rally for 250,000 people, which, unlike other events that we have given permits for in Central Park, won't fit in the park," Edward Skyler, press secretary to Mayor Michael R. Bloomberg, said in a statement.

The group, the statement continued, "has already agreed to a route which will take them right by Madison Square Garden. With less than three weeks to go, the organizers need to concentrate their efforts on making the necessary arrangements and working with the city to ensure a safe event and stop the theatrics."

Organizers said that they had submitted the new application to rally on the Great Lawn, North Meadow and East Meadow in the hopes that city officials would change their minds, after rejecting their original request for the Great Lawn, in part because of crowd size. But park officials deemed the request as being for "essentially the same event" and denied it.

Where the group goes from here is up in the air, organizers said. "We're not going to allow this infringement on basic civil liberties," said Bill Dobbs, the group's media coordinator, adding that it was considering suing over the park. "The way you hang on to rights is you exercise them, and it is very important to hang on to the right to assemble."

Still, Ms. Cagan said, the group did not have plans to encourage people to simply show up in the park if it is not permitted to rally.

And the group is not alone in its conflict over the prime swath of city green space. Lawyers from the Partnership for Civil Justice and the National Lawyers Guild plan to file a suit in Federal District Court in Manhattan later this week on behalf of the National Council of Arab Americans and the Answer Coalition, an antiwar and civil rights group. That suit stems from the denial of a permit to the council for a rally of 75,000 people on the Great Lawn, below its official capacity of 80,000, on Aug. 28.

But momentum appears to be building among other groups and individuals who are seeking to descend on the park, permit or not, on Aug. 29, as the Republicans arrive in the city. At the same time, another group, the A31 Action Coalition, is purposely not asking for permits for any of its demonstrations, some illegal, in part because of what they see as United for Peace and Justice's difficulties with the city.Indeed, the dispute over the protest site has become a central part of the message of some of the protest groups.

"The city's denial of Central Park must be called for what it is: the chilling move of an emerging police state," said Tanya Mayo, an organizer for Not in Our Name, an antiwar group that pressed for reopening the fight for the park. "Everyone who cares about the future of our humanity: Central Park, Aug. 29, is the place to be."