USA 'a little more perfect' after gay marriage legalised nationwide: Obama

President Barack Obama has hailed the legalisation of same-sex marriage throughout the USA, as conservative groups and states gear up in a fight to overturn the Supreme Court's decision.

In a 5-4 ruling, the highest court in the United States said the Constitution requires all 50 states to carry out and recognize marriages between people of the same sex.

Mr Obama praised the ruling as "a victory for America." The court decision marked a fresh coup for the White House, coming a day after the Supreme Court upheld an important and disputed section of Mr Obama's signature health care reform.

"Today we can say in no uncertain terms that we've made our union a little more perfect," Mr Obama said at the White House, which changed its online avatar to the rainbow colors of the gay rights movement.

"This decision affirms what millions of Americans already believe in their hearts -- when all Americans are treated as equal, we are all more free."

It also made the United States the 21st country or territory in the world that recognizes same-sex marriage as legal.

Flag-waving LGBT advocates on the packed Supreme Court forecourt -- some in tears -- cheered, danced, shouted "USA! USA!" and sang "The Star-Spangled Banner" in celebration.

Prominent in the crowd was Jim Obergefell, the lead plaintiff in the case, clutching a photo of his deceased husband John Arthur.

He took a brief phone call from Mr Obama, who told him: "Not only have you been a great example for people but you're also going to bring about a lasting change in this country.

"And it's pretty rare where that happens, so I couldn't be prouder of you and your husband. God bless you."

Mr Obergefell, who was live on television at the time, replied humbly: "Thank you, sir. That means an incredible amount to me."

Also in the vast crowd outside court as the life-changing news filtered through were Robert Westover, 51, and Tom Fulton, 57, who hugged and kissed.

"It feels like my birthday, the prom, our wedding day. It's hard to express the intensity of this moment, that our love now is equal," Mr Westover told AFP.

The case was brought by 14 same-sex couples, and the widowers of two gay couples, including Mr Obergefell, who had challenged de facto bans on same-sex marriage in Michigan, Kentucky, Ohio and Tennessee.

All four states had insisted in their respective constitutions that marriage could only be a union between a man and a woman.

"The Fourteenth Amendment (providing equal protection under the law) requires a state to license a marriage between two people of the same sex and to recognize a marriage between two people of the same sex when their marriage was lawfully licensed and performed out-of-state," the court ruling said.

Marriage has been a core institution in society since ancient times, "but it has not stood in isolation from developments in law and society," reasoned Justice Anthony Kennedy, who delivered the ruling.

To exclude them from marriage, Mr Kennedy said, would deny same-sex couples "the constellation of benefits that the states have linked to marriage."

Voicing dissent was Chief Justice John Roberts, who expressed concern that the court was making a decision better left to elected state legislatures.

"If you are among the many Americans - of whatever sexual orientation - who favor expanding same-sex marriage, by all means celebrate today's decision," he said.

"Celebrate the achievement of a desired goal. Celebrate the opportunity for a new expression of commitment to a partner. Celebrate the availability of new benefits.

"But do not celebrate the Constitution. It had nothing to do with it."

The decision came two years to the day after the Supreme Court, in another major ruling, struck down a controversial federal law that denied US government benefits to wedded gays and lesbians.

"This transformative triumph is a momentous victory for freedom, equality, inclusion, and above all, love," said Freedom to Marry, one of several groups battling for LGBT marriage rights.

"We'll remember this day for the rest of our lives," added the It Gets Better Project, another gay rights organization, in a mass email to supporters.

But the conservative Family Research Council expressed outrage, saying "no court can overturn natural law."

"Nature and nature's God, hailed by the signers of our Declaration of Independence as the very source of law, cannot be usurped by the edict of a court, even the United States Supreme Court," it said.

Texas Attorney General Ken Paxton, whose state has also prohibited same-sex marriage, said the fight going forward was now one of "religious liberty."

"No court, no law, no rule and no words will change the simple truth that marriage is the union of one man and one woman," he said in a statement.

The US Conference of Catholic Bishops, which champions the Vatican's opposition to marriage equality, called Friday's outcome "a tragic error."

"Jesus Christ, with great love, taught unambiguously that from the beginning marriage is the lifelong union of one man and one woman," it said.

With AFP.