"There is a concern here," Justice Sandra Day O'Connor told the city's lawyer, "for arbitrariness of the police."
At issue is a Chicago anti-loitering ordinance aimed at gang members and their friends. In the three years it was in effect before being struck down, the ordinance resulted in 45,000 arrests.
The court is expected to decide the case by July.
"Street gangs rely on their ability to terrorize the community," city lawyer Lawrence Rosenthal told the court. He said police officers too often arrive on the scene only to see gang members "pretending to innocently loiter."
"Gang crime has been rising," Rosenthal argued. "More people are getting shot."
But lawyer Harvey Grossman, representing 66 people arrested under the ordinance, called the disputed law "inherently vague and overbroad" and therefore unconstitutional.
The 1992 ordinance required police to order any group of people standing around "with no apparent purpose" to move along if an officer believed at least one of them belonged to a street gang. Those who refused to obey could be arrested.
"The question that is bothering some of us," said Justice David H. Souter, "is on what basis [people told to move on] can come to the conclusion that the police officer has the predicate to issue the order."
When Rosenthal argued that people do not need to be told beforehand why they should obey a police officer's order, Souter shot back, "Maybe they should and maybe not."
Rosenthal's most persistent questioner, Souter also said he had difficulty with what he called the ordinance's "silent assumption that some purposes are worthy and some are not."
"There usually is an apparent purpose for that which we call loitering," Souter said. One legitimate purpose could be to sit down and watch the cars go by, he said.
Justice Anthony M. Kennedy noted that the ordinance does not require police to tell people why they are being asked to move, and asked how anyone could know they are doing something unlawful.
Justices Ruth Bader Ginsburg and Stephen G. Breyer also voiced concerns over the law's broad sweep. Only Justice Antonin Scalia and Chief Justice William H. Rehnquist seemed sympathetic to the city's argument that the ordinance is a valid attempt to deal with "the enormous evils associated with gang loitering."
Scalia noted that governments have banned smoking in many public places, and asked, "Why is loitering more protected than smoking?"
He tried to goad Grossman into asserting a constitutional right to loiter, but Grossman did not.
Police say many of Chicago's gang members, estimated at more than 10,000, sell drugs from street-corner hangouts. Police blame gangs for 182 of the city's 759 homicides las year.
The Illinois Supreme Court struck down the ordinance last year, saying it restricted personal liberty while not distinguishing between innocent and harmful conduct.
In 1972, the nation's highest court struck down a Jacksonville, Fla., ordinance that prohibited "persons wandering or strolling around from place to place without any lawful purpose or object."
The Justices back then ruled that the ordinance was too vague because it did not give people enough notice of what conduct might be forbidden. Such ordinances were viewed by many as thinly veiled attempts to keep blacks out of certain towns or neighborhoods.
That same year, the Supreme Court upheld an ordinance that allows the arrest of someone who disobeys a police officer's order. That case involved a man who had gathered with others to interfere in a police investigation of a traffic incident, and police told him why he should leave.
Chicago's attempt to have its ordinance reinstated is being backed in friend-of-the-court briefs submitted by neighborhood organizations, the National League of Cities, the U.S. Conference of Mayors, the National Governors' Association, 31 states, and the Clinton administration.
Those urging the court to rule that the ordinance is unlawful include the NAACP and other civil rights groups, the National Law Center on Homelessness and Poverty, and the National Black Police Association.
By Richard Carelli