The proceedings began shortly after 9 a.m. The lawsuit challenges whether California's gay marriage ban, also known as Proposition 8, denies same-sex couples equal protection of the law under the U.S. Constitution.
The two gay couples on whose behalf the case was brought will be among the first witnesses in the trial, which is expected to last up to three weeks.
"And we're all Americans who simply want to get married just like everyone else. We believe in our Constitution and that the courts will lead the way to equality like they have so many times in the past. Thank you," said plaintiff Jeffrey Zarrillo.
Chief U.S. District Judge Vaughn R. Walker peppered both sides with questions during their opening statements.
Walker asked attorney Theodore Olson, who represents two same-sex couples suing to overturn Proposition 8, how the ban could be called discriminatory, since California already allows domestic partnerships.
Olson drew comparisons to some state laws banning interracial marriage in the 1960s that would have banned President Barack Obama's parents from getting married.
Charles Cooper, a lawyer for sponsors of the ban approved by voters in 2008, said in his opening statement that it's too difficult to know the impact of gay marriage on traditional marriage because the practice is still so new.
He urged the court to take a wait-and-see approach.
Acting less than two hours before the trial's scheduled start, the U.S. Supreme Court blocked the real-time broadcast that would have allowed the trial to be seen in other federal courthouses, at least for the first few days.
The high court also said it will not allow video of the trial to be posted on YouTube.com, even with a delay, until the justices have more time to consider the issue. It said that Monday's order will be in place at least until Wednesday.
Opponents of the broadcast say they fear witness testimony might be affected if cameras are present. They say those taking part in the trial could face retaliation and it could lead to a media circus.
Courtroom 3 at the U.S. Court of Appeals building in Pasadena was set up with two flat-screen TVs. As many as 125 people could have watched the trial live from San Francisco.
"I think actually this is a setback for the justice system as opposed to our community," said Robin Tyler, a Prop 8 opponent.
Tyler, who first sued for marriage equality in 2004 and later married her partner Diane Olson, arrived in Pasadena only to find out there would be no broadcast.
"Having cameras in the courtroom might have people tell the truth, maybe it will affect the witness testimony because they tried to say if marriage between same-sex couples passed we would go teach it in the schools and therefore go after their children. Maybe that's not the testimony they want people to hear," said Tyler.
Prop 8 opponents who planned to watch the opening arguments say they're disappointed, but not defeated.
"I am upset about the ruling today, but I also don't want to overreact. I'm willing to lose this battle if we win the war," said Brian Brookey, a Prop 8 opponent. "And I have every confidence that Judge Walker's going to, after looking at the evidence and hearing the arguments, make the right decision."
Thousands of gay and lesbian couples wed in the months before Prop 8 passed in November 2008. They are still considered legally married.
People on both sides of the issue will be watching the case very closely.
"What happened with Proposition 8 was I believe unconstitutional, I believe it was undemocratic and I think it was wrong. and I'm hoping, in fact I'm confident, that eventually we'll end up on the right side of history," said Rev. Susan Russell, All Saints Episcopal Church.
"We're committed to defending the seven million California voters who twice voted in favor of Prop 8. They want marriage to remain as society has always defined it," said Dr. La Verne Tolbert, Protect Marriage Coalition.
As expected, protestors on both sides of the issue are making their voices heard outside the federal courthouse in San Francisco. The demonstrations have been peaceful.
The attorneys who represented former Vice President Al Gore and former President George W. Bush in the Florida recount in 2000 are joining forces to argue in support of gay marriage.