PHOENIX (Reuters) - A group of same-sex couples in Arizona filed a federal lawsuit on Thursday to challenge the state's ban on gay marriage, arguing that it violates the equal protection clause of the U.S. Constitution, court documents showed.

The lawsuit, filed in U.S. District Court in Phoenix by seven same-sex couples and two people whose same-sex partners had died, was the latest of a series of legal actions across the nation challenging such state bans.

"Every day that same-sex couples in Arizona are denied marriage, the government sends a message that their families are not worthy of equal dignity and respect," Jennifer Pizer, one of the attorneys who filed the legal action, said in a statement.

The lawsuit comes as gay rights advocates gain momentum in their fight to legalize same-sex marriage. Federal judges have recently struck down gay marriage bans in states including Utah and Texas. Such rulings have been put on hold pending appeals.

The trend follows a U.S. Supreme Court ruling in June that legally married same-sex couples nationwide are eligible for federal benefits. The decision struck down a key part of the 1996 federal Defense of Marriage Act.

Arizona is among more than 30 states that still ban gay or lesbian couples from marrying. The lawsuit says an Arizona constitutional amendment and state statutes that ban same-sex marriage violate federal equal protection and due process clauses.

Lead plaintiff in the case are Nelda Majors,75, and Karen Bailey, 74, a gay couple living in Scottsdale, Arizona, who have been together for 55 years and have raised two children together.

(Reporting by David Schwartz; Writing by Cynthia Johnston; Editing by Scott Malone and Andrew Hay)