LOS ANGELES  – Sean Penn won the best actor Oscar on Sunday for his portrayal of slain San Francisco gay rights activist Harvey Milk in the movie "Milk."

Penn, 48, picked up the second Oscar of his career, the first being his 2004 win for his lead role as a grieving father in "Mystic River."

"I did not expect this and I wanted to be very clear that I know how hard I make it to appreciate me," Penn said as he accepted the award.

His portrayal of an openly gay politician was a timely one -- with "Milk" coming out shortly after California's same-sex couples lost their right to marry in a voter referendum.

Penn, who is not gay and has a tough guy image dating back to his early career scrapes with the paparazzi, convincingly evoked the legendary charm of Milk, who was shot to death in 1978 at San Francisco City Hall.

"He came in kind of ready made" for the role, openly gay "Milk" director Gus Van Sant told Reuters.

Several critics called the performance the best of Penn's already storied and versatile career, which took off after his breakthrough surfer-stoner role as Spicoli in "Fast Times at Ridgemont High."

This year's best actor race was thought to be a battle between Penn and Mickey Rourke, an actor who scored a remarkable career comeback as an aging athlete trying to remain in the ring in "The Wrestler."

Penn, one of the most respected actors in Hollywood, won the Screen Actors Guild award for best actor earlier this year and a slew of critics' prizes. The Oscar nomination for "Milk" was his fifth in the best actor category.