OKLAHOMA CITY (AP) - Supporters of a proposed initiative petition to grant ``personhood'' rights to human embryos have asked the U.S. Supreme Court to reverse a ruling by the Oklahoma Supreme Court that stopped the ballot measure.

Personhood USA filed the petition with the nation's highest court on Monday. It asks justices to allow Oklahoma citizens to circulate and collect signatures on a personhood initiative petition.

The ballot measure was challenged by the American Civil Liberties Union and the New York-based Center for Reproductive Rights on behalf of several Oklahoma doctors and residents.

The state Supreme Court ruled unanimously in April 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.''