Gay rights groups cheered the announcement by US President Barack Obama that he supports same-sex marriage, but conservatives swiftly denounced his landmark stance.
"Congratulations, Mr. President, for making history today by becoming the first sitting president to explicitly support marriage for same-sex couples," said Rea Carey, executive director of the National Gay and Lesbian Task Force.
"Who benefits? Millions of families who now know that their country's leader believes in fairness for all. This is a great day for America," said Carey in a statement.
"We celebrate this moment and also remember that the right of loving, committed couples to get married is just one of many issues affecting lesbian, gay, bisexual and transgender people," she added.
"It is heartening to know the president stands with our families in the pursuit of full equality, economic security and justice."
Brian Brown, president of the National Organization for Marriage, which actively campaigns against same-sex marriage, said Obama had now made the definition of marriage "a defining issue" in the upcoming elections.
"God is the author of marriage, and we will not let an activist politician like Barack Obama who is beholden to gay marriage activists for campaign financing to turn marriage into something political that can be redefined according to presidential whim," he said in a statement.
"President Obama stuck a fork in himself today. He's done. He's toast," said Bryan Fischer of the American Family Association, who linked the president's stance with his looming electoral showdown with Republican hopeful Mitt Romney.
"What God has defined, man may not redefine," Fischer said. "President Obama today came out on the wrong side of history, morality and public policy. For him, it is a day that will live in infamy."
Cardinal Timothy Dolan, the president of the US Conference of Catholic Bishops, called Obama's comments "deeply saddening."
Evan Wolfson, founder and president of Freedom to Marry, which campaigns for same-sex marriage, said Obama's support "marks a turning point for the freedom-to-marry movement."
"Yet there is much left to be done," he warned. "It is time to repeal discriminatory laws that hurt families and help no one and speed passage of freedom to marry laws throughout the country."
He added: "Government has no business putting obstacles in the path of loving and committed couples and their families who simply seek to care for one another and for whom marriage matters."
"This is a major turning point in the history of American civil rights," said Mayor Michael Bloomberg of New York, home to one of the biggest gay and lesbian communities in North America.
"The march of freedom that has sustained our country since the Revolution of 1776 continues, and no matter what setbacks may occur in a given state, freedom will triumph over fear and equality will prevail over exclusion," he said.
"Today's announcement is a testament to the president's convictions, and it builds on the courageous stands that so many Americans have taken over the years on behalf of equal rights for gay and lesbian Americans."
In Maryland, Governor Martin O'Malley, who signed legislation in March to legalize same-sex marriage, said "the way forward is always to be found through greater respect for the equal rights and human dignity of all."
"Ultimately, we all want the same thing for our children: to live in a loving, stable committed home protected equally under the law," he said.
Six states plus the national capital Washington DC have legalized gay marriage. Two others, Washington state and Maryland, have voted in favor, but their laws have yet to come into effect pending referendums.
"Thank you President @BarackObama for your beautiful and brave words. I'm overwhelmed," comedienne Ellen DeGeneres, who wed her partner in California in 2008 before the state reversed its decision on gay marriage, said on Twitter.