The Supreme Court ruled yesterday that police may sometimes stop and frisk a person who runs at the mere sight of an officer, in a unanimous decision that gives police more leeway on the streets.

In a case arising against a backdrop of public fear about crime as well as concerns over aggressive police tactics, the court refused to adopt a blanket rule that a person can be stopped simply because he runs away after seeing the police. That position was advocated by the state of Illinois in the case and endorsed by 17 other states.

Instead, the court said that such a reaction can be an important factor in determining whether police have a "reasonable suspicion" adequate for stopping a suspect. The decision expanded the court's decades-old rules about when police can stop and search people without a specific reason to believe a crime has occurred but did not break significant new ground.

"Headlong flight--whenever it occurs--is the consummate act of evasion," Chief Justice William H. Rehnquist wrote for the court. "It is not necessarily indicative of wrongdoing, but it is certainly suggestive of such."

But Justice John Paul Stevens, joined by three other justices, emphasized that some people might have innocent reasons for dodging police. "Among some citizens, particularly minorities and those residing in high crime areas, there is . . . the possibility that the fleeing person is entirely innocent, but . . . believes that contact with the police can itself be dangerous," Stevens wrote in an opinion signed by Justices David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer.

Those four justices joined the chief's opinion allowing flight to be a major factor in whether individuals are stopped but dissented on his resolution of the case of a Chicago man who bolted when he saw four police cars approach.

The majority said officers had sufficient reason to stop William Wardlow, noting that he was also in an area of heavy narcotics trafficking and carrying a white bag. Stevens said those factors were "too generic and susceptible to innocent explanation."

The touchstones for the case, Illinois v. Wardlow, were the Fourth Amendment, which prohibits "unreasonable" searches, and the justices' 1968 ruling on situations in which police are suspicious of an individual but lack specific grounds to believe a crime has occurred. In Terry v. Ohio, the court said police can stop and frisk someone if--after considering all the circumstances of the scene--an officer has reason to believe he is dangerous.

Wardlow, who was caught carrying an illegal handgun after officers pursued him in the 1995 incident, argued at trial that officers lacked the grounds to pursue and search him. An Illinois trial court disagreed, but the state Supreme Court ruled that his sudden flight was not enough to justify a stop.

Reaction among defendants' rights groups was mixed. George Kendall of the NAACP Legal Defense and Educational Fund said he was concerned about police targeting minorities but expressed relief that the court did not adopt the broad Illinois position. "The law is largely left where it was before," he said.

But Tracey Maclin, a Boston University law professor who had filed a "friend of the court" brief on behalf of the American Civil Liberties Union and other groups, said the ruling will lead police to use flight as an excuse for chasing individuals who rightfully fear police.