The letter from the presumptive Democratic presidential nominee was read Sunday at the annual Pride Breakfast of the Alice B. Toklas LGBT Democratic Club in San Francisco, the Sacramento Bee reported.

In the past, Obama has said that states should be free to decide on gay marriage. In the letter, he said he supports equal rights for gays and lesbians on both the state and federal level.

"And that is why I oppose the divisive and discriminatory efforts to amend the California Constitution, and similar efforts to amend the U.S. Constitution or those of other states," Obama wrote.

The state Supreme Court ruled recently that gays have a right under the state constitution to marry. The first weddings under the ruling were held in June.

Obama's Republican counterpart, Sen. John McCain, supported an initiative in Arizona to define marriage as heterosexual. But he voted against amending the U.S. Constitution to ban gay marriage.