A three-judge panel of the 9th U.S. Circuit Court of Appeals issued the injunction two days after a federal judge upheld the ban and threw out a lawsuit brought against the Republican-backed law, which was due to go into effect on Thursday.

The measure bars doctors from performing abortions after 20 weeks of pregnancy except in a medical emergency, defined as a case where an immediate abortion is required to avert the mother's death or a risk of "substantial and irreversible impairment of a major bodily function."

Under the law, physicians found in violation of the ban would face a misdemeanor criminal charge and possible suspension or revocation of their licenses.

Abortion-rights advocates have cited the Arizona law as the most extreme version of late-term abortion prohibitions recently enacted in more than half a dozen states. Such restrictions have emerged as one of the latest new abortion battlegrounds since Republicans gained greater statehouse political clout in the November 2010 elections.

Arizona already bans abortions at the point of viability, when the fetus could potentially survive outside the womb, generally considered to begin at 22 to 24 weeks of pregnancy. A full-term pregnancy typically is about 40 weeks.

The U.S. Supreme Court legalized abortion in 1973 but allowed states to place restrictions on the procedure from the time of viability unless the woman's health was at risk.

Abortions after 20 weeks of gestation are rare. In Arizona, pregnancies terminated at 21 weeks or more accounted for just 77 of the 11,059 total abortions performed statewide in 2010, according to the latest figures available from the state Department of Health Services.

The American Civil Liberties Union and the Center for Reproductive Rights challenged the Arizona statute this month in a suit believed to be the first court case brought on behalf of abortion providers testing the constitutionality of such laws.

DEBATED MEDICAL RESEARCH

Six states have put laws into effect in the past two years banning late-term abortions, based on hotly debated medical research suggesting a fetus feels pain starting at 20 weeks of gestation. North Carolina enacted its own such ban decades ago.

Arizona and two other states have adopted similar laws that have yet to take effect.

U.S. District Judge James Teilborg had ruled on Monday that the Arizona measure, signed into law in April by Republican Governor Jan Brewer, was consistent with limits federal courts have allowed to be placed on late-term abortions.

He denied a request for an injunction to halt the law and threw out the case on its merits, saying the ban "does not impose a substantial obstacle" to abortions generally and that Arizona had the right to enact such a measure.

He also found that the state had provided "substantial and well-documented" evidence that a fetus has the capacity to feel pain during an abortion by at least 20 weeks of development.

The ruling prompted the appeal to the San Francisco-based 9th Circuit.

"We are relieved that the court blocked this dangerous ban and that women in Arizona will continue to be able to get safe, appropriate medical care," ACLU staff attorney Alexa Kolbi-Molinas said in a statement.

Under the two-page order issued by the appeals court, attorneys have until mid-October to present their briefs, and then the case will be placed on the first available argument calendar for a hearing.

"The order to place the case for argument recognizes the important interests of Arizona, and I look forward to the opportunity to counter the misplaced arguments of the plaintiffs," Maricopa County Attorney Bill Montgomery, who argued the case before Teilborg, said in a statement.

(Additional reporting by Dan Whitcomb in Los Angeles; Writing by Steve Gorman; Editing by Cynthia Johnston and Lisa Shumaker)

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