The Rev Jesse Jackson has called for California’s ban on gay marriage to be struck down.
The Baptist minister and civil rights activist spoke at a gathering outside the San Francisco appeals court where the latest hearings on the ban are taking place.
Rev Jackson has previously opposed Proposition 8 but stopped short of supporting gay marriage.
But in a speech on Monday, he said: “Marriage is based on love and commitment – not on sexual orientation. I support the right for any person to marry the person of their choosing.”
The gathering outside the Ninth Circuit Court of Appeals courthouse was organised by Marriage Equality USA.
The court heard arguments earlier this week on whether California’s same-sex marriage ban is constitutional. A federal judge ruled in August that it was not but gay marriage opponents appealed.
Rev Jackson told supporters that gays and lesbians were “America’s newest second-class citizens”.
He said: “We stand together today to uphold the principles of due process, of equal protection under the law, of fighting against discrimination against any and all people based on race, religion, gender or sexual orientation.
“We stand with you today to support marriage equality, and to declare that Proposition 8 must be struck down as unconstitutional.
“Peoples’ right to self-expression, self-determination be respected and affirmed. It’s time to challenge ignorance, a time to break the silence and the chains of hatred, of divisive and discriminatory bigotry.
“Marriage is based on love and commitment – not on sexual orientation. I support the right for any person to marry the person of their choosing.
“If Dr King and our civil rights movement has taught us anything, it’s the fundamental principle of that all people deserve equal protection under the law.
“LGBT people deserve equal rights – including marriage equality – and equal protection under the law. Discrimination against one group of people is discrimination against all of us. The state – and the courts – should not sanction discrimination.”
Molly McKay, Marriage Equality USA’s media director, said Rev Jackson’s remarks were “truly extraordinary”.
She said: “We are deeply touched and honored to have Rev Jesse Jackson, one of the most powerful civil rights leaders alive today, join our event today and make these historic remarks in support of marriage equality.
“Hearing Rev Jackson’s vision of the Rainbow Coalition as I came of voting age in the late ‘80s inspired me to want to get involved in making our country’s promise of equality and justice for all ring true.
“Hearing his vocal support against Prop 8 and in support of marriage equality today his been a truly extraordinary and we look forward to continuing to work together towards our shared vision of forging a One Big Tent America.”