Throughout Saturday evening, police remained in constant contact with organizers of the Occupy Chicago demonstrations, warning that anyone remaining in Grant Park after the 11 p.m. curfew, violating a city ordinance, risked being hauled off to jail, city officials said.
"The mayor made it clear that he wanted protesters to have ample warning that the park was closing as well as time to leave should they choose to do so," Mather said.
Occupy Chicago organizer Kelvin Ho said about 500 protesters who had camped out at the park Saturday night were divided into color-coded groups — red for those who did not want to be arrested, yellow for those who were undecided and green for those who were willing to remain behind and go to jail.
At 12:45 a.m., McCarthy made the call for police to move in and arrest anyone remaining in the park. Shortly after 1 a.m., police began leading 175 people away in handcuffs.
"Demonstrators were in violation of the law, and it is the obligation of police to enforce the law," Mather said.
Police officials determined they could not allow the more than 2,000 protesters to spend the night in the public park because it would be harder to get them out in the coming days, according to a police source familiar with the events.
It also would set a bad precedent for dealing with thousands of demonstrators expected to converge on Chicago from around the world during the G-8 and NATO summits that will be held simultaneously in May, the source said.
The arrests were a stark contrast to the clash that occurred in New York two weeks ago when nearly 700 protesters were arrested on the Brooklyn Bridge.
Chicago police maintained open lines of communication with the group throughout the protest and extended ample time and opportunity for the group to leave the park, said Lt. Maureen Biggane, a police spokeswoman. The group largely complied, she said.
"Police provided repeated notification of the need to comply with the Park District ordinance," Biggane said. "Approximately 90 minutes after the park was officially closed and remaining members made it clear they were not going to disperse in accordance with the ordinance, arrests were made."
The Occupy Chicago protests, one of many that have sprouted up around the world as an outgrowth of the Occupy Wall Street movement, could be viewed as a small-scale practice run for Emanuel, whose leadership skills will be on international display next spring.
The last thing Chicago wants, according to political analysts, is a repeat of the violent confrontations that occurred during the 2000 G-8 summit in Seattle. Chicago also still has the stigma of the bloody riots at the 1968 Democratic National Convention that tainted Mayor Richard J. Daley's image.
"The city administration, with a new police chief and a new mayor, is learning on the job how to deal with these kinds of events," said Dick Simpson, who heads the political science department at the University of Illinois at Chicago. "The fact that the arrests were orderly rather than filled with anger and confrontation shows that things are more positive now."
There is less potential for violence, Simpson said, because the Occupy Chicago movement is about the financial breaks that have been given to Wall Street investors and the very rich, not local police or city government.
"The tension is not there now. But whether it will be there with the G-8 is another issue," Simpson said.
On Sunday night, several hundred Occupy Chicago protesters peacefully marched from LaSalle Street and Jackson Boulevard back to Grant Park. The rally ended with protesters quietly filtering into downtown about 8:45 p.m. There were no arrests reported, according to police.
Emanuel previously has expressed empathy for the Occupy Chicago protesters. During an editorial board meeting with the Tribune last week, he said that although he does not "agree with the methodology or all the claims," he understands the frustration.