Los Angeles, California (CNN) -- A federal judge ruled on Thursday to allow same-sex couples to marry in California, starting on August 18, handing another victory to supporters of gay rights in a case that both sides have said is likely to end up in the U.S. Supreme Court.

Chief U.S. District Judge Vaughn Walker in San Francisco struck down the state's ban on same-sex marriage last week, ruling that voter-approved Proposition 8 violates the U.S. Constitution. Walker had issued a temporary stay on his decision, which on Thursday he said he would lift.

The high-profile case is being watched closely by supporters and opponents of same-sex marriage, as many say it is likely to make its way to the U.S. Supreme Court. If it does, the case could result in a landmark decision on whether people in the United States are allowed to marry others of the same sex.

Same-sex marriage is legal in five U.S. states and in the District of Columbia, while civil unions are permitted in New Jersey. The five states are Massachusetts, Connecticut, Vermont, Iowa and New Hampshire.

Same-sex couples in California were permitted to marry, briefly, before Proposition 8 passed in 2008.

"Today's ruling means that in less than one week, equality under the law will be restored for millions of loving families across California," said Rick Jacobs, founder of the Courage Campaign.

Dozens of same-sex couples gathered outside City Hall in San Francisco ahead of Walker's decision, hoping they would be allowed to marry. Many waved rainbow flags and carried signs that read: " 'I DO' support the freedom to marry." The crowd erupted in cheers after the judge's decision, though couples there must now wait at least another week or so to get married.

Proposition 8 is part of a long line of seesaw rulings, court cases, debates and protests over the controversial issue of same-sex marriage. Proposition 8 defines marriage as a union between a man and woman and passed in California with some 52 percent of the vote in November 2008.

Opponents of same-sex marriage have said their best bet lies with higher courts and have appealed the federal judge's ruling. The case heads next to the 9th U.S. Circuit Court of Appeals before possibly going to the U.S. Supreme Court.

By allowing his stay to remain in place until August 18, Walker gave supporters of Proposition 8 more time for the 9th U.S. Circuit Court of Appeals to weigh in on the case. The appeals court could allow same-sex marriages to move forward, or it could put them on hold indefinitely as the case winds its way through the legal system.

"Judge Walker is in fact making victims out of these people who might now go get married and then not know what the status of their relationship is," said Tom McClusky, a leader of the legislative arm of the Family Research Council, which does not support same-sex marriage.

People in the United States are split over the idea of allowing same-sex couples to marry. Forty-nine percent of respondents said they think gay and lesbian couples have the constitutional right to marry and to have their marriages recognized by law, while 51 percent said those rights do not exist, according to a new CNN/Opinion Research Corporation Poll.