The first couple to begin filling out the paperwork was Marcia Hams, 56, and her partner for 27 years, Susan Shepherd, 52, of Cambridge. They showed up at midnight Saturday - a full 24 hours ahead of time - to stake out the first spot in line where the city clerk was to hand out the nation's first state-sanctioned gay marriage applications.
"I'm shaking so much," Hams said as she filled out the application while sitting at a table across from a city official. "It's just amazing that this day is here, we just never thought it would ever happen!."
Outside, throughout the day and into the night, the atmosphere was festive - complete with a giant wedding cake - as officials in the liberal bastion of Cambridge seized the earliest possible moment to begin the process of granting same-sex couples the historic right at the center of legal battles nationwide.
The state's highest court had ruled gays and lesbians must be allowed to marry beginning Monday, and some of the couples in line planned to head to the courts as soon as they opened later in the morning to seek waivers allowing them to wed before the usual three-day waiting period.
No one expects the marriage licenses being issued in Massachusetts to be treated as a private matter.
Promised a waiver of the normal three-day waiting period, the seven gay and lesbian couples who successfully sued for marriage rights in Massachusetts will wed before relatives, friends and supporters in Boston and three other towns.
The couples' jubilation will be shared by gay-rights advocates across the country, including many in states such as New York, California, Washington and New Jersey where comparable lawsuits are moving forward.
"This isn't just one historic moment in Massachusetts," said Kevin Cathcart, executive director of the gay-rights group Lambda Legal. "It's the start of what will be a long period of progress and breakthroughs, with gay couples in other states also winning the right to marry."
For foes of gay marriage, Monday's weddings represent a stinging defeat - but one they hope will be reversed by a backlash among politicians and voters nationwide.
"What I'm starting to see is people who are apolitical, who never got involved before, saying, 'This is too much - we don't want same-sex marriage foisted on us,"' said Mathew Staver, president of a Florida-based legal group, Liberty Counsel, that is opposing gay marriage in numerous court cases.
"There will be a federal challenge," Constitutional law expert Roger Pilon told CBS News Correspondent Sharyn Alfonsi.
That challenge is expected after same-sex couples, married in Massachusetts, cross state lines and seek benefits in other states.
Pilon says that is the point when "the objection will be raised by the state that 'We don't recognize your marriage.'"
The Massachusetts Legislature has approved a proposed amendment to the state constitution that would ban gay marriages but legalize Vermont-style civil unions. But the earliest the measure could be put before the voters is November 2006. President Bush is also championing a federal constitutional ban on gay marriage.
On Friday, the U.S. Supreme Court declined to block the state's municipal clerks from issuing marriage licenses to gay couples, clearing the way for Monday's weddings.
The stay had been sought by a coalition of state lawmakers and conservative activists.
A staunch opponent of same-sex marriage, Republican Governor Mitt Romney pointed to an old state law limiting licenses to state residents only, reports CBS News Correspondent Mika Brzezinski. "We have to follow the law whether it's one we appreciate or not," he said of the court's decision to allow gay marriage.
CBS News Legal Analyst Andrew Cohen says what happens in Massachusetts - the way the marriage licenses are issued, and the effect they have in Massachusetts, will impact the legal case as it goes forward.
Both sides in the debate expect the issue to figure prominently in the November election, with Massachusetts serving as a rallying cry and alarm bell.
Candidates for Congress will face pressure to explain their position on a proposed federal constitutional amendment that would ban gay marriage. Voters in Georgia, Kentucky, Mississippi, Oklahoma, Missouri and Utah - and probably several other states - will consider similar amendments to their state constitutions.
"It will be a national referendum about gays and gay marriage," said Rod McKenzie of the National Gay and Lesbian Task Force. "We're the underdog when it comes to all these ballot measures - the scale is bigger than we've ever had to deal with."
In states with the ballot measures, divisive campaigns already are underway.
An Oklahoma gay-rights group, for example, took out newspaper ads last week showing an outline of the state with "Closed" stamped over it. The ad contended that businesses would leave - or stay away - if voters approved the constitutional ban on gay marriage.
State Sen. James Williamson, a Republican from Tulsa, called the ad outrageous and predicted that a ban would attract new businesses.
"There is a real hunger for a return to traditional values and for leaders who will draw a line in the sand to help stop the moral decay of this country," he said.
Nationwide, both sides are planning marches and rallies over the coming week - among them, pro-gay marriage events in Iowa City, Iowa, and Las Cruces, New Mexico, and a "Not on My Watch" rally in Arlington, Texas, for pastors opposed to gay marriage.
Even if many Americans wish otherwise, Massachusetts, as of Monday, will join the Netherlands, Belgium and Canada's three most populous provinces as the only places worldwide where gays can marry, though the rest of Canada expected to follow soon.
In the Netherlands, which pioneered gay marriage three years ago, the practice now stirs little controversy. Cheryl Jacques, a former Massachusetts legislator who now heads the Human Rights Campaign, a major gay-rights group, hopes her compatriots eventually emulate the Dutch.
"For the vast majority of Americans, Monday will be a completely ordinary day - nothing's going to change," she said. "But for some Americans in Massachusetts - gay and lesbian families - it will be a truly historic day, when their families will be made stronger and their children will become safer."
"I'm very proud of my state," Jacques added. "Massachusetts is going to teach the rest of the country a lesson - equality doesn't hurt anyone."