The Supreme Court yesterday ruled that police officers do not have the right to shoot unarmed suspects fleeing from nonviolent crimes.

The 6-to-3 ruling struck down "shoot-to-kill" laws in nearly half the states that allow the police to shoot suspects on the run, no matter what the severity of the crime. The ruling involved an unarmed 15-year-old youth in Memphis, Tenn., who was shot to death in 1974 as he tried to run from an unoccupied house from which he had stolen two $5 bills and a purse. The court, in an opinion by Justice Byron R. White, said it is unconstitutional for police to use deadly force unless the officer has reason to believe the suspect poses a significant threat of death or serious harm to the officer or to others.

In other action, the court:

* Strengthened the ability of cities and towns to defend themselves from lawsuits alleging violations of antitrust laws.

* Divided evenly on whether a community can be required to allow a Christmas Nativity scene on public land.

"The use of deadly force to prevent the escape of all felony suspects, whatever the circumstances, is constitutionally unreasonable," White said in the "deadly force" ruling, even if failure to shoot means a suspect would escape.

It was common to allow the use of deadly force against fleeing felons in earlier times, White said, but that was when almost all felonies were punishable by death.

The American Civil Liberties Union said the court's ruling was overdue. "We couldn't be more pleased that the court finally bit the bullet and ruled that police officers cannot execute people prior to trial," Charles Sims, national staff counsel for the ACLU in New York, said.

The ruling was also hailed by the International Association of Chiefs of Police in Gaithersburg.

Many states and most large cities have moved in recent years toward more restrictive laws on police use of force, generally allowing officers to shoot only when there is a danger to themselves or the public.

The rule sanctioned by the court yesterday follows that trend but is not as restrictive. It also would allow officers to shoot suspects believed to have committed violent crimes, even if they were not armed when police spotted them.

But "where the suspect poses no immediate threat to the officer and no threat to others," White said, "the harm resulting from failing to apprehend him does not justify the use of deadly force.

"It is no doubt unfortunate when a suspect who is in sight escapes," White said, "but the fact that the police arrive a little late or are a little slower afoot does not always justify killing the suspect. A police officer may not seize an unarmed, nondangerous suspect by shooting him dead."

Under the Memphis city code, failure to heed a policeman's warning to stop is punishable by a $50 fine, White said. "Thus, [the youth's] attempted escape subjected him to (a) a $50 fine, and (b) being shot."

"It is not better that all felony suspects die than that they escape," White said.

Justice Sandra Day O'Connor, dissented from the ruling, saying she could not "accept the majority's creation of a constitutional right to flight for burglary suspects seeking to avoid capture at the scene of the crime."

O'Connor, joined by Chief Justice Warren E. Burger and William H. Rehnquist, said "burglary is a serious and dangerous felony," and the "public interest in the prevention and detection of the crime is of compelling importance."

The shooting in Memphis was "deeply regrettable," O'Connor said.

But the officers' actions in this case were reasonable, she said. "To avoid the use of deadly force and the consequent risk to his life, the suspect need merely obey the valid order to halt," she said.

The reasonableness of the officers' actions "is not determined by the unfortunate nature of this particular case," O'Connor wrote. "Instead, the question is whether it is constitutionally impermissible for police officers, as a last resort, to shoot a burglary suspect fleeing the scene of the crime."

The Memphis youth's father sued public officials in 1975 for violating his son's civil rights. After lengthy delays and rehearings, a federal appeals court in 1983 struck down the Tennessee deadly force law.

The high court yesterday noted that the police officer is no longer a defendant in the suit and the state is immune from suit, but sent the case back to lower courts to determine whether Memphis and its police department must pay damages in the death of the youth.