The Supreme Court, extending constitutional limits on police power to question defendants, ruled yesterday that police may not try to obtain a confession once a defendant has been charged and has requested a lawyer.

The court, in an opinion by Justice John Paul Stevens, said the Sixth Amendment right to "assistance of counsel" means that, if a defendant asks for a lawyer, there can be no valid waiver of those rights if police attempt further questioning.

The 6-to-3 ruling in Michigan v. Jackson came in cases involving two men convicted of separate murders. Both men requested lawyers at their arraignments before magistrates. Before the lawyers arrived, police questioned the men again.

The police read them their rights as required under the court's 1966 Miranda ruling and obtained incriminating statements. The Michigan Supreme Court threw out the statements, saying that the police nevertheless violated both men's Sixth Amendment rights.

Stevens, agreeing with the Michigan court, extended a 1981 opinion, Edwards v. Arizona, which bars police from questioning a suspect who has asserted his Fifth Amendment rights to remain silent and to have a lawyer.

"The Sixth Amendment right to counsel [after arraignment] requires at least as much protection as the Fifth Amendment right to counsel at any custodial interrogation," Stevens said, extending a consistent line of Sixth Amendment cases guarding the rights of those who have been formally charged.

Ronald J. Bretz, an attorney for one of the men, said the ruling expands Miranda rights, which apply only when a suspect is in custody. The Sixth Amendment right upheld yesterday, he said, "kicks in after someone is charged and applies whether the defendant is in custody or at home or at any time. . . . "

Both men must now be freed or retried. Bretz said it is almost certain that both will be retried.

Justice William H. Rehnquist, writing in dissent with Justices Lewis F. Powell Jr. and Sandra Day O'Connor, said there is no need for new protections of a defendant's right to a lawyer and that the majority "provides no satisfactory explanation for its decision to extend the Edwards rule."

In a second decision yesterday, the court said federal labor law forbids cities and states from intervening in private labor disputes.

Justice Harry A. Blackmun, writing for the court in Golden State Transit Corp. v. Los Angeles, said Los Angeles acted improperly when it refused to renew a taxi company's franchise until the company settled a strike by its drivers.

"The settlement condition imposed by the Los Angeles City Council," Blackmun said, "destroyed the balance of power designed by Congress, and frustrated Congress' decision to leave open the use" of each side's economic power to pursue its interests.

Blackmun, overturning an appeals court ruling in favor of the city, said the city's action in 1981 was preempted by the National Labor Relations Act.

Rehnquist, in lone dissent, said Congress never intended to justify the majority's conclusion that "a city, not seeking to place its weight on one side or the other of the scales of economic warfare," could not act in order to let the company "put its taxis back on the streets where the franchise presumably contemplated they would be."