The first US federal court case to determine whether states are allowed to ban same-sex marriages has opened in San Francisco, California.

Any ruling reached is expected to be challenged, possibly taking the case all the way to the US Supreme Court.

A Supreme Court ruling would determine the fate of gay marriages nationwide, without the possibility of appeal.

The suit, filed by two gay couples, challenges Proposition 8 - a ban on gay marriage in the state of California.

The law amended California's constitution to restrict marriage to a union between a man and a woman.

Supporters of the challenge are comparing it to landmark cases which ended segregation in US schools and overturned a ban on interracial marriage, the BBC's Rajesh Mirchandani reports.

They say the Constitution enshrines the right to marry but, by limiting it to heterosexual couples, it discriminates against gay people.