Demonstrators, civil rights advocates and the police say they view the protest, against the occupation of Iraq, as a test of how all sides will handle demonstrations during the Republican National Convention late this summer.

The police said the pens would increase safety, help crosstown traffic flow, and enable them to use fewer officers at big events. "It's not to create a hostile environment," said Paul J. Browne, the chief spokesman for the department.

Protest organizers say the pens are more appropriate for controlling cattle than people, and they point out that countless demonstrations in New York and elsewhere have been held peaceably without them.

"They're very confusing for people," said Leslie Cagan, the national coordinator for United for Peace and Justice, the organizers of the March 20 event. "Am I allowed into the pen? Am I allowed out of the pen? Who decides when they're open, when they're closed?"

Mr. Browne said that the pens, which would be closed one by one as they fill with people, would be used only during the rally, not the march, and that people would be allowed to leave at will. He said the department would post directions for protesters on its Web site. "While they're there they can go
to our recruitment page if they're interested in joining the department," he deadpanned.

United for Peace and Justice was responsible for a major antiwar
demonstration in February 2003 in which demonstrators complained that the police used charging horses, unprovoked arrests and a maze of barricades to control the crowd, preventing some people from leaving and blocking tens of
thousands from reaching the demonstration site.

Police Commissioner Raymond W. Kelly gave the police high marks for its handling of that event, which was attended by more than 100,000 people. He blamed problems on a lack of cooperation from organizers, who he said failed to advertise the correct location or provide the promised number of marshals.

After that demonstration, the New York Civil Liberties Union filed suit to prevent the police from using perimeter barricades and pens, dispersing crowds with horses without notice, and searching demonstrators. The group's fears that the police will use the threat of arrest to discourage protesters
were heightened this week when Robert M. Morgenthau, the Manhattan district attorney, said his office was told to expect 1,000 arrests a day at the convention.

"They should be figuring out how not to arrest 1,000 people, not how to arrest 1,000 people," said Christopher Dunn, the associate legal director of the civil liberties union.

The police and the demonstration organizers also disagreed yesterday about arrangements. Mr. Browne said demonstrators would enter Madison Avenue at 42nd street and walk down to assemble for the march. Ms. Cagan said they were also telling people to approach from the north, but from 34th Street.

One issue, however, was resolved: unlike last year, the police agreed to allow portable toilets at the rally.