America's top court has convened to take up the delicate and divisive issue of same-sex marriage.
Nine Supreme Court justices will consider the legality of a California ballot initiative that limits marriage to opposite-sex couples.
As sign-carrying demonstrators from both sides gathered peacefully in front of the Supreme Court building on a brisk morning in Washington, the first of two days of oral arguments on the issue got under way inside.
Tomorrow, the court will consider the 1996 federal Defense of Marriage Act (DOMA), which limits the definition of marriage to opposite-sex couples.
Rulings in both cases are expected by the end of June.
In what were scheduled to be about three hours of deliberations with lawyers over the two days, the justices will have their say on what gay activists see as a civil rights issue reminiscent of famous Supreme Court cases of the past, including Loving v Virginia, a 1967 case in which the court invalidated bans on interracial marriage.
The cases come before the high court at a time when more states have legalised gay marriage.
Last year three more - Maryland, Maine and Washington - did so, bringing the total to nine plus the District of Columbia.
"Never before in our history has a major civil rights issue landed on the doorstep of the Supreme Court with this wave of public support," said Theodore Boutrous, a lawyer for opponents of the California intitiative, which is known as Proposition 8.
Strong opposition to gay marriage still exists, however, both among Republicans in Congress and in many states.
A total of 30 states, including California, have constitutional amendments that ban gay marriage. Nine states, including California, recognise civil unions or domestic partnerships among same-sex couples.
Outside the courthouse, about 1,000 protesters waved signs, danced to music and traded arguments.
The crowd was largely pro-gay marriage, though a large group of opponents was expected to arrive later in the morning.
Among others who planned to demonstrate outside was Chief Justice John Roberts' cousin Jean Podrasky, a lesbian from California who would like to marry her partner.
Some experts said that, with the issue unsettled in the states, a majority of the justices might be disinclined to make sweeping pronouncements as the democratic process plays out.
There are various ways they could do that in June - when a ruling is expected - as the Proposition 8 case presents multiple options for the justices.
For instance, they could proclaim that gay marriage bans are constitutionally unsound. They could uphold Proposition 8 as a law with a legitimate purpose approved by a majority of voters in California.
They could also plot a middle path by striking down the law without making broad pronouncements about whether gay marriage bans in other states should be struck down.
Another way the court could rule later this year might be viewed as an anticlimax of sorts: The justices could simply decide that they cannot rule on the merits because of the procedural complexities that brought the case to the high court.
The state of California declined to support Proposition 8 when the plaintiffs filed suit in 2009 in a federal District Court in San Francisco, meaning there was no party defending the law until its proponents entered the case.
The federal judge struck the law down, a ruling that was upheld by the San Francisco-based 9th US Circuit Court of Appeals.
A Supreme Court decision concluding that the law's backers do not have legal standing to defend the law would wipe out the Appeals Court decision, but leave the District Court decision that struck down Proposition 8 on the books.
The way the justices rule could depend in large part on the likely swing voter, Justice Anthony Kennedy.
Although a conservative appointed by President Ronald Reagan, Kennedy has in the past authored two opinions that expanded gay rights.
Lawyers representing two same-sex couples in California who want to marry are hoping the justices will go big and are making the most sweeping arguments.
The counsel for Kris Perry and Sandy Stier and Paul Katami and Jeff Zarrillo will argue that under the US Constitution's equal protection guarantee, there is a fundamental right for people to marry someone of the same sex.
Ms Perry, who has raised four children with her partner Stier, was hopeful and optimistic.
"We have been waiting for a long time to get married," she said last week. "We are very excited to have the end in sight."