By Ronnie Cohen

SAN FRANCISCO (Reuters) - A Christian legal group has filed a federal lawsuit challenging a landmark California law that bars a controversial therapy aimed at reversing homosexuality from being used on children and teens, calling it a violation of privacy and free speech rights.

California's Democratic Governor Jerry Brown signed the ban into law over the weekend, making the nation's most populous state the first to ban so-called conversion therapy among youth. Gay rights advocates say the therapy can psychologically harm gay and lesbian youth.

"This legislation is an outrageous violation of the civil rights of youth, of parents and of licensed counselors, including clergy who are licensed counselors," said Brad Dacus, president of the Pacific Justice Institute, which filed the suit late on Monday for a student who underwent the therapy and two counselors.

"What we're advocating is for all to have the freedom and liberty to seek the counseling that meets their needs," Dacus told Reuters by telephone.

Enactment of the law marked a major victory for gay rights advocates who say the treatment, also called reparative therapy, has no medical basis because homosexuality is not a disorder.

The measure prohibits therapists from performing sexual-orientation change counseling with children and teens under 18 and was supported by the California Psychological Association and the California Board of Behavioral Sciences, among others.

The bill's sponsor, state Senator Ted Lieu, a Democrat from the Los Angeles suburb of Torrance, called the lawsuit frivolous.

"Under the plaintiffs' argument, the First Amendment would shield therapists and psychiatrists from medical malpractice and psychological-abuse claims simply because they use speech in practicing their medicine. That is a novel and frivolous view of the First Amendment," he said.

LAW CALLED A GOVERNMENT INTRUSION

The suit says the law violates numerous provisions of the U.S. Constitution, including the right to privacy, freedom of speech and the free exercise of religion.

"The statute involves government intrusion into an intimate zone of privacy," the suit says, adding that one of the plaintiffs, Dr. Anthony Duk, frequently treats patients with a combination of counseling and prescription drugs, using the medication to help his clients control their sex drives.

Kate Kendell, executive director of the National Center for Lesbian Rights, derided the lawsuit and said she would work with the state attorney general's office to defend the law, which is scheduled to take effect in January.

"It's a series of very artful dodges and red herrings and has really zero merit," she told Reuters. "This is a law that protects minors from a practice regarded by every mainstream mental health organization as harmful, damaging and without any basis in scientific fact."

Representatives of the attorney general were not immediately available for comment. Meanwhile, another Christian group said it also planned to file a suit to block the law.

Mat Staver of the Florida-based Liberty Counsel said his group planned to sue the state on behalf of counselors as well as two parents and their minor children who are currently undergoing the therapy in California.

"This new law has just gone way beyond common sense," Staver told Reuters. "Counselors cannot counsel minors about reducing or eliminating same-sex attraction even if the client is asking for this counseling."

Dr. Robert Spitzer, the psychiatrist who pioneered the treatment, has since renounced it and has apologized to the gay and lesbian community.

(Editing By Cynthia Johnston and Cynthia Osterman)