The Oklahoma Supreme Court on Monday halted an effort to grant "personhood" rights to human embryos, saying the measure is unconstitutional.

The state's highest court ruled unanimously that a proposed amendment to the Oklahoma Constitution that would define a fertilized human egg as a person violates a 1992 U.S. Supreme Court decision involving a Pennsylvania case and "is clearly unconstitutional." Supporters of the personhood amendment are trying to gather enough signatures to put it before Oklahoma voters on the November ballot.

Opponents contend the measure would ban abortions without exception and interfere with a woman's right to use certain forms of contraception and medical procedures, such as in vitro fertilization.

The American Civil Liberties Union and the New York-based Center for Reproductive Rights filed a protest with the state Supreme Court on behalf of several Oklahoma doctors and residents. They asked the court to stop the group Personhood Oklahoma from gathering signatures.

The nine-member court determined the initiative petition "is void on its face" and struck it down.

"The only course available to this court is to follow what the United States Supreme Court, the final arbiter of the United States Constitution, has decreed," the court said.