The U.S. Supreme Court heard more than an hour of arguments Tuesday on a California law banning same-sex marriage - the first of two cases being heard this week that could open the door to gay marriage nationwide.

The court's nine justices are considering whether California's Proposition 8 violates the U.S. Constitution's equal protection clause. During Tuesday's session, Justice Anthony Kennedy, seen as the "swing vote" in the case, raised concerns about the court entering "uncharted waters" by throwing out the California ban.

The justices will make their decision about the law later this year.

On Wednesday, the high court will consider the federal Defense of Marriage Act, which defines marriage as being only between one man and one woman.

Supporters and opponents of gay marriage have been gathering outside the court for days, hoping to get a coveted seat to hear the cases.

Both sides also are marching and rallying in Washington.

Recent public opinion polls show a majority of Americans now support same-sex marriage. Nine states plus the District of Columbia have legalized the practice. Supporters call it a human and civil-rights issue and are hoping for a decision similar to one in 1967 that struck down state laws banning interracial marriage.

But 29 other states have passed amendments in their constitutions that outlaw gay marriage. Opponents insist the institution of marriage must be protected to ensure the family unit.