The controversial ballot measure was launched in response to a state supreme court decision allowing gays and lesbians to wed. The proposition passed with 52 percent of the vote in November 2008.
"With a stroke of his pen, Judge Walker has overruled the votes and values of 7 million Californians who voted for marriage as one man and one woman," said Brian Brown, president of the National Organization for Marriage, in a statement Wednesday.
Lawyers supporting the ballot measure had argued that voters endorsed a "fundamental, definitional feature" of marriage that has historical roots "in this country and, almost without exception, in every civilized society that has ever existed."
But Walker found the plaintiffs in the case -- one lesbian and one gay couple -- demonstrated by "overwhelming evidence" that Proposition 8 violates their rights to due process and equal protection under the Constitution.
"Moral disapproval alone is an improper basis on which to deny rights to gay men and lesbians. The evidence shows conclusively that Proposition 8 enacts, without reason, a private moral view that same-sex couples are inferior to opposite-sex couples," wrote Judge Walker.
Geoff Kors, executive director for Equality California, which filed an amicus brief supporting the Prop. 8 federal challenge, say gay couples across the state were "thrilled" by the ruling. "Judge Walker has preserved our democracy by ruling that a majority cannot deny a minority group of fundamental freedoms," he said.
The verdict will now be appealed to the Ninth Circuit Court of Appeals and may ultimately reach the U.S. Supreme Court, where experts say a decision could transform social and legal precedent. They compare the potential impact to the famous 1954 Brown v. Board of Education decision, which desegregated schools, and the 1967 Loving v. Virginia decision, which ended laws banning interracial marriage.
Five states -- Massachusetts, Iowa, Connecticut, Vermont and New Hampshire -- and the District of Columbia currently perform same-sex marriages. Four states recognize marriages performed elsewhere, and nine states grant civil unions or partnerships.