The justices, by a 6-3 vote, upheld a 1993 Colorado ``bubble'' law that bars people from counseling, distributing leaflets or displaying signs within eight feet of others without their consent whenever they are within 100 feet of a health clinic entrance.
``This statute simply empowers private citizens entering a health care facility with the ability to prevent a speaker, who is within 8 feet and advancing, from communicating a message they do not wish to hear,'' Justice John Paul Stevens wrote for the court.
Violators can get up to six months in jail and a $750 fine.
The law had been challenged as a violation of protesters' free-speech rights.
``The right to free speech, of course, includes the right to attempt to persuade others to change their views, and may not be curtailed simply because the speaker's message may be offensive to his audience,'' Stevens wrote. ``But the protection afforded to offensive messages does not always embrace offensive speech that is so intrusive that the unwilling audience cannot avoid it.''
His opinion was joined by Chief Justice William H. Rehnquist and Justices Sandra Day O'Connor, David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer.
Dissenting were Justices Antonin Scalia, Anthony M. Kennedy and Clarence Thomas. Scalia and Kennedy both read from their dissents on the bench, totaling about 20 minutes.
Scalia called the ruling ``one of many aggressively pro-abortion novelties announced by the court in recent years.''
``Today's decision is an unprecedented departure from this court's teachings respecting unpopular speech in public'' areas, Kennedy's dissenting opinion said.
The decision marked the first time the nation's highest court reviewed a state legislature's attempt to regulate anti-abortion demonstrators outside health clinics. The justices twice previously had ruled in abortion-protest disputes but those had stemmed from court injunctions aimed at specific clinics, not laws with statewide effect.
In 1994, the court upheld a 36-foot demonstration-free zone around a Florida clinic's entrances but struck down provisions for a larger buffer zone. And the justices in 1997 upheld a court-ordered 15-foot buffer zone around New York clinic entrances but struck down a judge's call for a ``floating buffer zone'' requiring protesters to stay at least 15 feet away from people as they entered or left the clinics.