BOSTON – Launching the next battle in the legal war over gay marriage, eight couples and 12 municipal clerks said Thursday they will file lawsuits challenging the 1913 law used to block out-of-state same-sex couples from marrying in Massachusetts.

The groups, including a couple from Cape Elizabeth, Maine, said they would file two lawsuits in Suffolk Superior Court on Friday, claiming the law, which bars couples who live outside of Massachusetts from marrying if their marriages would be illegal in their home states, is unconstitutional and that it is discriminatory to enforce it against same-sex couples.

Gov. Mitt Romney and Attorney General Tom Reilly cited the law – which some say was originally written to be used against interracial couples – in denying marriage licenses to out-of-state gay couples.

After gay marriage became legal in Massachusetts last month, several city and town clerks openly defied them and issued the documents to nonresidents anyway, until Reilly ordered them to stop or face penalties.

“The governor simply can’t dust off this law to discriminate against gays and lesbians,” Michelle Granda, a lawyer for Gay and Lesbian Advocates and Defenders, which represents the eight out-of-state couples, said at a news conference Thursday.

Romney’s office referred questions to Reilly’s office, which declined comment. In his letter to clerks ordering them to stop issuing the licenses, Reilly said doubts by public officials about the constitutionality of the law were not grounds to stop enforcing it.

The state’s Supreme Judicial Court ruled last November that same-sex couples had a right under the Massachusetts Constitution to marry, and ordered that weddings could begin taking place May 17.

The couples come from the five other New England states and New York. Some have already gotten married in Massachusetts, while others applied for licenses and were denied.

Michael Horne, 51, and James Theberge, 46, of Cape Elizabeth, Maine, have been together 21 years and became parents together last year when Massachusetts granted them joint adoption of their son, Nate, now 2.