* Anti-abortion protesters fought law on free-speech grounds
* Supreme Court liberal Kagan among justices critical of law
* Ruling on Massachusetts case expected by end of June (Recasts lead, adds details from arguments, paragraphs 4, 10-12, 14-16, 19)
WASHINGTON, Jan 15 (Reuters) - U.S. Supreme Court justices expressed doubt on Wednesday about a Massachusetts law that mandates a protective buffer zone around abortion clinics to allow patients unimpeded access, indicating they may strike it down as unconstitutional as demanded by anti-abortion protesters.
Challenging the 2007 law, anti-abortion activists say it violated their freedom of speech rights under the First Amendment of the U.S. Constitution by preventing them from standing on the sidewalk and speaking to people entering the clinics.
During a one-hour argument before the high court, a majority of the nine justices expressed concern that the law may be too broad. One justice, Elena Kagan, asked questions indicating she felt the size of the 35-feet (11 meter) no-entry zone for protesters around clinics was simply too big.
A narrow ruling overturning the law as too broad could give the state the opportunity to enact a new, less-restrictive statute.
One of the main concerns raised by justices was the law does nothing to distinguish between protesters seeking to disrupt a clinic's work and people who say they wish to quietly counsel women and try to persuade them not to have abortions.
It is not clear if the court would have the votes to go further than striking down the Massachusetts law and possibly overturn a 2000 Supreme Court precedent that upheld a similar buffer-zone law in Colorado.
Of the current nine justices, five were on the bench when that case, Hill v. Colorado, was decided.
The Massachusetts law was enacted in part because of safety concerns highlighted by violent acts committed against abortion providers in the past. In 1994, two abortion clinic workers were killed outside a clinic in Brookline, Massachusetts.
Among those critical of the Massachusetts law was Kagan, from the liberal wing of the court. At one point she noted the Massachusetts law "does have its problems."
Kagan's main concern appeared to be the size of the buffer zone.
"I guess I'm a little bit hung up on why you need so much space," she told Massachusetts' lawyer, Jennifer Miller.
Justice Stephen Breyer, another of the liberal justices, pressed Miller over whether state legislators, when debating the law, had sought to distinguish between people who wish to conduct "calm conversations" and those who are more disruptive. Miller said the evidence showed that even those attempting peaceful conversations could create congestion in front of the facility.
'NOT A PROTEST CASE'
Justice Antonin Scalia, one of the most conservative members of the court, in particular seemed sympathetic to the plaintiffs in the case, who he said should not be described as "protesters" because they only desire to speak quietly to women entering the clinic.
"This is not a protest case," he said. "These people don't want to protest abortion. They want to talk to women who are about to get abortions and try to talk them out of it."
The zone "might not be so bad" if it addressed only protesters, Scalia added.
Along similar lines, Justice Anthony Kennedy told Miller, that when enacting laws that restrict one type of speech, "you have a duty to protect speech that is lawful."
Scalia and Kennedy were both in the minority in the 2000 Colorado case.
Not all the justices appeared as skeptical about the Massachusetts law.
Justice Ruth Bader Ginsburg focused mainly on the history of violence at abortion clinics and previous state efforts to stop disruptions, which the state says had not worked.
She noted that the state "doesn't know in advance who are the well-behaved people and who are the people who won't behave well."
Ginsburg and Breyer were both in the majority in the 2000 case.
Justice Clarence Thomas, who was a dissenter in the 2000 case and would be expected to hold similar views now, did not speak during the argument, as is his custom. Chief Justice John Roberts, who was not on the bench when the earlier case was decided, said nothing as well.
The case specifically concerns people who want to protest outside three Planned Parenthood facilities that offer abortions in addition to other health services for women in Boston, Springfield and Worcester.
Two other states, Montana and Colorado, have similar laws. Municipal ordnances and court injunctions have also been used in various states to create similar buffer zones.
A ruling is expected by the end of June.