PORTLAND, Ore. -- A U.S. federal judge on Monday struck down a voter-approved ban on gay marriage in the northwestern state of Oregon, saying it is unconstitutional, marking the 13th consecutive legal victory for gay marriage advocates since last year's U.S. Supreme Court ruling that overturned part of a federal ban.

U.S. District Judge Michael McShane joined judges in seven other states who have struck down gay marriage bans this year, though appeals are underway and the issue is expected to settled by the U.S. Supreme Court. He said the ban unconstitutionally discriminates against same-sex couples and ordered Oregon not to enforce it. State officials earlier refused to defend the constitutional ban in court.

Jubilant couples rushed to tie the knot following Monday's ruling, including some who stood in line at the Portland county building for hours to get a marriage license.

The U.S. Supreme Court last year struck down the 1996 Defence of Marriage Act, which barred the federal government from recognizing same-sex marriage. The court determined the law improperly deprived gay couples of due process. Many observers predicted the ruling would create a pathway for states to act, as polls showed a majority of Americans now support gay marriage. Indeed, lower-court judges have repeatedly cited that decision when striking down bans.

In addition to Oregon, 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. At least 17 other states have filed lawsuits asking judges to throw out state bans.

The two most recent states to make the unions legal were New Mexico and Hawaii, both of which did so in late 2013. Oregon's ruling is not expected to be challenged, which would make it the 18th state where gay marriage is legal.

Opposition remains stiff in many places. Critics note most states still do not allow gay marriage and that in most of those that do, it was the work of courts or legislatures, not the will of the people.

In Oregon, 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 McShane's decision.

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.

Democratic Attorney General Ellen Rosenblum refused to defend the ban, saying there are 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, and says she won't appeal.

The judge denied a request by the National Organization for Marriage to defend the law on behalf of its Oregon members. A panel of the 9th U.S. Circuit Court of Appeals on Monday refused the group's request for an emergency stay of that decision, allowing same-sex marriages to proceed.

Gay rights groups previously said they've collected enough signatures to force a statewide vote on gay marriage in November. But they said they would discard the signatures and drop their campaign if the court rules in their favour by May 23.

Oregon law has long prohibited same-sex marriage, and voters added the ban to the state constitution in 2004. The decision, approved by 57 per cent of voters, came months after Multnomah County, the state's largest, briefly issued marriage licenses to same-sex couples.

About 3,000 gay couples were allowed to marry before a judge halted the practice. The Oregon Supreme Court later invalidated the marriages.

A ruling from a federal appeals court is expected soon, either from a panel in Denver reviewing rulings from Utah and Oklahoma or judges in Virginia reviewing that state's case. Many legal observers say they expect the U.S. Supreme Court to take a case at some point, but they acknowledge it's impossible to predict what the high court will do. The Supreme Court could also just wait and see how U.S. appellate courts rule. It often waits until there is a conflict between appellate courts before taking a case.