Boston ? Gay rights activists sued on Tuesday in an effort to block a proposed constitutional amendment that would put an end to same-sex marriage in Massachusetts.

The lawsuit, filed by Gay and Lesbian Advocates & Defenders, challenges a September ruling by state Atty. Gen. Tom Reilly that found the amendment drive was legal.

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

Gary Buseck, GLAD legal director, said the Massachusetts Constitution bars any citizen-initiated amendment that “relates to the reversal of a judicial decision.”

A Reilly spokesman defended the decision.