Gay marriage supporters filed a lawsuit Tuesday against Attorney General Tom Reilly in an effort to block a proposed ballot issue that would amend the state Constitution to outlaw same-sex marriage.

The lawsuit was filed by Gay and Lesbian Advocates & Defenders and challenges a September ruling by Reilly that the repeal effort is legal.

That ruling allowed backers of the proposed amendment to begin collecting signatures. They gathered more than 120,000 certified signatures -- well above the 65,000 needed to get the measure on the 2008 ballot.

Gary Buseck, the group's legal director, said the state Constitution specifically bars any citizen-initiated amendment that "relates to the reversal of a judicial decision." Only the Legislature can propose that kind of amendment, he said.

Reilly should have blocked the question from going forward on those grounds, Buseck said.

"This proposed anti-gay, anti-marriage amendment is meant squarely and solely to reverse the decision in Goodridge v. Dept. of Public Health that ended marriage discrimination in Massachusetts," Buseck said, referring to the landmark 2003 court decision legalizing same-sex marriage.

A spokesman for Reilly did not immediately return a call seeking comment.

A spokeswoman for the Massachusetts Family Institute, which is campaigning for the gay marriage ban, did not immediately have a comment on the lawsuit.

The GLAD lawsuit was filed with a single justice of the Supreme Judicial Court on Tuesday and could be passed along to the full court.