Two lesbians were among the first same-sex couples to wed under new rules in the US state of Massachusetts.
There were emotional scenes as gay couples applied for certificates
In the coming days, hundreds of couples are set to follow the example of Marcia Kadish and Tanya McCloskey of Malden, who have been together for 18 years.
The issue has divided opinion and politicians across the US.
President George W Bush repeated his call to Congress to amend the US Constitution defining marriage "as a union of a man and a woman".
"The sacred institution of marriage should not be redefined by a few activist judges," Mr Bush said.
On Friday, the US Supreme Court rejected a last-ditch attempt by conservative groups to stop same-sex marriages from becoming legal in the state.
There were jubilant scenes in the city of Cambridge, which began issuing licences to gay and lesbian couples from midnight (0400 GMT on Monday).
Gay and lesbian couples queued for marriage licences outside the city hall - which opened as the law came into force.
Newly-wed Tanya McCloskey said: "It was really important to us to just be married... Part of it is, we don't know what the Legislature is going to do."
She and partner Marcia Kadish worked at breakneck pace to fill out paperwork, get a waiver from the usual three-day waiting period, then return to city hall - where they got their marriage licence and exchanged vows.
Other towns and cities across the state were also prepared to wed large numbers of same-sex couples as the law came into force.
The Supreme Court ruling upheld a decision by the state's highest court.
It said that denying marriage licences to same-sex couples violated anti-discrimination laws.
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The Massachusetts ruling has fuelled heated debate across the country - and the controversy has been particularly intense in an election year.
In a statement, President Bush said he had called on the Congress "to pass, and to send to the states for ratification, an amendment to our Constitution defining and protecting marriage as a union of a man and a woman as husband and wife".
"The need for that amendment is still urgent, and I repeat that call today."
His rival John Kerry - who is a Massachusetts senator - is also opposed to same-sex marriages, but favours a more limited form of legal recognition.
Thousands of same-sex couples were married in San Francisco earlier this year, but the marriages were not recognised by the state of California.
A mayor in New York state is being prosecuted after performing gay marriages in February.