The US Supreme Court treaded cautiously on Tuesday as it considered gay marriage, with justices appearing hesitant to deliver a sweeping historic verdict on the emotionally charged issue.
On a day long awaited by both sides of the debate, thousands of gay rights advocates descended on the Supreme Court, many waving US and rainbow flags, outnumbering a rally of religious activists opposed to same-sex marriage.
The Court will take months to issue a ruling. But the majority of the justices lean conservative and several indicated that the court would be in no hurry to extend the right to same-sex marriage to the entire country.
"It seems to me we have an institution and you don't have to include everybody," Chief Justice John Roberts, who is generally considered a conservative, said during the first of two days of arguments.
Justice Samuel Alito, also a conservative, said the court needs to be cautious about same-sex unions as marriage "is essential for the preservation of society."
Pundits closely watched comments from Justice Anthony Kennedy, who is often seen as the swing vote on the nine-judge bench.
Kennedy appeared to question the breadth of the case, playing down calls for a sweeping ruling. But he also voiced concern for the rights of children of same-sex couples, saying they want full recognition for their parents.
Nine states and the District of Columbia allow same-sex marriage. Some rights advocates hope that the Supreme Court will pave the way to guarantee gay marriage across the United States, sidestepping legislative fights.
The top court is hearing two cases. The first concerns the constitutionality of Proposition 8, in which California voters in 2008 ended the right to same-sex marriage in the nation's most populous state.
Theodore Olson, a prominent lawyer who is leading the charge for same-sex marriage, declined to predict how the court would rule from the justices' comments.
Olson said that justices could issue a "very broad, sweeping conclusion with respect to the rights of our citizens" or issue a narrower decision that pertains only to California.
Sandy Stier, one of plaintiffs in the case, said that the voters' referendum had hurt her family by invalidating her marriage. She and her partner, Kris Perry, have four sons.
"I, like all Americans, believe in equality. I also believe in our judicial system and I have great faith in it. But more than anything, I believe in love," Stier said on the steps of the Supreme Court.
"It is our hope that we can move forward and remove this harm from society so that gays and lesbians in California can go back to their lives living equally alongside their neighbors with the same rights and protections as everyone else," she said.
On Wednesday, the court will consider the Defense of Marriage Act, a federal law which defines marriage as an act between a man and woman.
The star plaintiff in that case is Edie Windsor, 83, who had to pay $363,000 in federal estate taxes when her partner of more than 40 years, Thea Spyer, whom she had married in Canada in 2007, died in 2009.
Under the law, the surviving member of a heterosexual married couple is exempt from such taxes.
Outside the Supreme Court protesters from both sides of the debate set out their positions.
One demonstrator dressed in pink and danced to music of Lady Gaga, the superstar singer and supporter of gay rights.
Among opponents, protesters pushed strollers and chanted: "Every child deserves a mom and dad."
Opinion polls have repeatedly indicated that most Americans accept the principle of same-sex marriage, including an overwhelming majority of younger Americans.
President Barack Obama -- who last May became the first US president to publicly back gay marriage -- restated his support for it on Monday through a Twitter account managed in his name by an advocacy group.