BARNSTABLE, Mass. (AP) – City and town clerks will not have to demand documentary proof of residency from gay and lesbian couples who wish to marry in Massachusetts under new state guidelines that appear more lenient than originally proposed.

Gov. Mitt Romney, who opposes same-sex marriage, had originally said clerks would have to seek documents proving gay and lesbian couples lived in Massachusetts before granting them marriage licenses on May 17, when same-sex marriage becomes legal.

His top legal adviser told clerks, however, that a couples’ sworn oath about where they live and about the truth of the information they provide on their marriage license will suffice as proof of residence.

Attorney Daniel Winslow warned city and town clerks, however, that issuing marriage licenses to out-of-state gay and lesbian couples will make the marriages null and void and could result in legal repercussions for the clerks.

The consequences of an illegal marriage could be severe for the couple, particularly if they have children, because of legal questions of support and custody, Winslow said. There also would be legal consequences for the clerks, though Winslow declined to say what those could be.

“What conscientious clerk ever would issue a license in violation of Massachusetts law when the consequences and ramifications to children and innocent parties would be so great?” Winslow said in an interview with The Associated Press. “I simply can’t see that any clerk would do that.”

A 1913 Massachusetts law bars out-of-state couples from marrying here if their marriage would be illegal in the state where they live.

After months of legal wrangling over the Supreme Judicial Court’s ruling legalizing same-sex marriage in Massachusetts, state and local officials are getting into the nitty-gritty of how to put this first-in-the-nation law into practice.

The state’s 351 city and town clerks will be on the front lines, dispensing the new licenses, which make no distinction between gay and straight couples.

Several clerks have taken steps to have more workers on hand on the first day to process the expected influx of couples, while Cambridge has announced plans to open the doors of city hall just after midnight to issue the state’s first licenses.

Before the first licenses are issued, Cambridge officials will have a celebration, including a wedding cake.

In Boston, the state’s largest city, about 20 city workers wearing “welcome” badges will be on duty at City Hall to answer questions. The city is printing about 3,000 color brochures, including a letter of congratulations from Mayor Thomas Menino.

In Northampton and Provincetown, towns with large gay and lesbian populations, clerks will not be holding special hours, but plan to expand operations during the day to handle the high volume.

On May 4, the state held the first of five training sessions for clerks. At the closed-door session in Barnstable, 70 clerks saw the state’s new intention-to-marry forms, which have been altered to remove gender references and now include a place to show what proof of residence was provided.

Clerks said they were satisfied with the level of detail they received from the governor’s office on how to implement the new law, and relieved they won’t have to ask for documentary proof of residency.

“To me, the oath is the strongest piece of evidence we can possibly get,” Plymouth Clerk Laurence Pizer said. “Happiness is clarity and the ability to go forward and I think we can do that now.”

Several clerks, including those in Provincetown, Worcester, Northampton and Lowell, have resisted Romney’s previous edict that they ask for proof of residency from gay and lesbian couples. In the past, all couples have had to do is sign an oath, under penalty of perjury, that there is no impediment to their marriage.

While that sworn oath will still be legally acceptable, Winslow said, the state Department of Public Health plans to advise clerks that written proof is the best documentation.

Clerks and gay-rights advocates welcomed the news that the sworn oath would continue to suffice, even if that is not Romney’s preferred choice.

“This has been a practice that up until now has not been widely followed,” Winslow said. “But Massachusetts marriage laws have never been so broad compared to the rest of the nation before.”

Winslow instructed clerks to be consistent – to ask all couples for proof of residence, not just gay and lesbian couples. Only out-of-state gay and lesbian couples, however, could be subsequently barred from marriage due to the 1913 law.

Attorney Mary Bonauto, who represented seven same-sex couples in the landmark case that led to the legalization of same-sex marriage, predicted a legal challenge of the residency requirements but said that the state’s instruction to clerks appeared to be a concession to their concerns about becoming “marriage police.”

“It’s important that they have acknowledged that even though the governor’s office wants documentary proof of residence, they’re going to give clerks leeway to do what clerks have always done, which is accept the oaths,” Bonauto said.