The group planning the largest protest during the Republican National Convention agreed yesterday to hold a giant rally along the West Side Highway, acceding to the demands of the Bloomberg administration, which opposed the group's effort to demonstrate in Central Park.

"We are not happy about this," said Leslie Cagan, national coordinator for United for Peace and Justice, an antiwar umbrella group. "The clock is ticking; we need to move on," she added. "So, we decided to take the high road here."

More than a year ago, the group began seeking a permit for 250,000 people to rally on the Great Lawn in Central Park on Aug. 29, the day before the convention begins. But the Parks Department rejected that site, saying that the area could not hold that many people and that a huge rally would damage the lawn.

Instead, police officials suggested that demonstrators mass somewhere between 14th and 23rd Streets on Seventh Avenue. From there, they could march north past Madison Square Garden, where the convention will be held, head west on 34th Street to 12th Avenue and then south along the highway to around Chambers Street, where a soundstage for the rally would be set up. Protest organizers and police officials estimate the crowd could stretch as far north as 34th Street.

Protest organizers originally balked at that plan. But in accepting the city's offer yesterday after failed attempts to sway the administration, they conceded that they had been outflanked by officials, who delivered a public ultimatum last week to accept the Hudson River site or take the city to court, which could have led to a long battle with an uncertain outcome.

"In terms of the rally location, we got nothing," Ms. Cagan said, adding that she hoped the raft of public support for a rally in the park would somehow benefit the group in gaining other concessions from the city. "We have a body of experience, we actually know what we're talking about when we go into these meetings," she said. "That should help."

But the acceptance of the highway proposal now brings to the forefront a host of details officials and organizers must hash out.

Chief among them is where officials would place an emergency vehicle lane, how barricades will be configured and how people will be able to join or leave the protest. In addition, organizers say they are concerned about the use of surveillance video cameras and police helicopters, which can make it difficult for demonstrators to hear.

Organizers have asked that the city consider helping to defray the cost of equipment, which Ms. Cagan said would run at least $150,000 more than it would have in Central Park. In addition, organizers are asking the city to help provide access to things like water and transportation, since the new location is far from mass transit and lacks the shade of Central Park.

"If the weather on August 29th is anything like what it was like today, then water is actually a public health issue and a public safety issue," Ms. Cagan said, adding that her group would like to discuss these questions directly with the mayor's office, which has thus far delegated the negotiations to the Police Department.

Paul J. Browne, the Police Department's chief spokesman, said the police were "proceeding now with the details to accommodate a safe and peaceful demonstration, including the deployment of barriers as appropriate." He added, "The amenities cited by the organizers are customarily supplied by the demonstrators or organizers themselves."

And there was little indication from the mayor's office that the custom was likely to change. "We're not in the business of providing lunches or sound systems or transportation for permitted events," said Edward Skyler, the mayor's press secretary. "I think they see some sort of political value in having the city tell them no, so they keep coming up with preposterous demands."

Organizers have argued that the city is providing amenities like free MetroCards to the Republican delegates, but Mr. Skyler said they were being paid for with private donations.

If the protesters "want to find corporate sponsors for their protests, let them go ahead," he said. "New Yorkers shouldn't have to see their tax dollars spent on subsidizing protests."