U.S. District Judge Michael McShane threw out the voter-approved ban Monday.
State officials have said they'd be prepared to carry out same-sex marriages almost immediately, and couples lined up outside the county clerk's office in Portland in anticipation of the decision.
Four gay and lesbian couples brought suit arguing Oregon's marriage laws unconstitutionally discriminate against same-sex couples and exclude them from a fundamental right.
State officials refused to defend the ban, and McShane earlier denied a request by the National Organization for Marriage to intervene on behalf of its Oregon members.
An appeals court Monday morning refused the group's request for an emergency stay of McShane's decision.
THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.
A federal judge was expected to make Oregon the latest state to allow gay marriage Monday after state officials refused to defend its constitutional ban in court.
Gay couples were poised to tie the knot immediately after a federal appeals court denied a last-minute request to block the judge's impending ruling that could strike down the state's same-sex marriage ban.
Officials in Oregon's largest county, Multnomah, said they'll begin issuing marriage licenses immediately if U.S. District Judge Michael McShane's decision allows it. McShane hasn't signaled how he'll rule, but both sides in the case have asked that the voter-approved ban be found unconstitutional.
The judge last week denied a request by the National Organization for Marriage to defend the law on behalf of its Oregon members. On Monday morning, the group appealed that denial to the 9th U.S. Circuit Court of Appeals, seeking an emergency stay of the decision.
But the appellate court quickly denied the group's request, clearing the way for same-sex marriages to begin immediately if a McShane's ruling allows it.
In Portland, couples lined up outside the county clerk's office in anticipation of a favorable decision.
Laurie Brown and Julie Engbloom arrived early Monday at the Multnomah County Building to form the line for marriage licenses. The two have been a couple for 10 years. Engbloom proposed in April, when they celebrated their anniversary by climbing Smith Rock in Central Oregon.
"We always knew we wanted to spend our whole life together," Brown said. "This opportunity has come, it feels right, everything has fallen into place."
Seventeen states and the District of Columbia allow gay marriage. Federal or state judges in Idaho, Oklahoma, Virginia, Michigan, Texas, Utah and Arkansas recently have found state same-sex marriage bans to be unconstitutional. Judges also have ordered Kentucky, Ohio and Tennessee to recognize same-sex marriages from other states.
But opposition remains stiff in many places. Critics point out that most states still do not allow gay marriage and that in most that do, it was the work of courts or legislatures, not the people.
Four gay and lesbian couples brought the Oregon cases, arguing the state's marriage laws unconstitutionally discriminate against them and exclude them from a fundamental right to marriage.
In refusing to defend the ban, Democratic Attorney General Ellen Rosenblum said there were no legal arguments that could support it in light of decisions last year by the U.S. Supreme Court. She sided with the couples, asking the judge to overturn the ban.