Philip Seymour Hoffman, one of the leading actors of his generation and winner of an Academy Award for his title role in the film "Capote," was found dead in his Manhattan apartment on Sunday in what a New York police source described as an apparent drug overdose.

Hoffman, 46, was discovered unresponsive on the bathroom floor of his Greenwich Village apartment by police responding to a 911 call, and Emergency Medical Service workers declared him dead on the scene, New York City police said in a statement. An investigation was under way.

A police spokesman said investigators found Hoffman with a syringe in his arm and recovered two small plastic bags in the apartment containing a substance suspected of being heroin. A police department source earlier told Reuters that Hoffman had died of an apparent drug overdose.

Hoffman, who is survived by three children with his partner Mimi O'Donnell, had detailed his struggles with substance abuse in the past.

"We are devastated by the loss of our beloved Phil and appreciate the outpouring of love and support we have received from everyone," Hoffman's family said in a statement issued through his publicist.

"This is a tragic and sudden loss and we ask that you respect our privacy during this time of grieving. Please keep Phil in your thoughts and prayers," it added. A representative said the family would not make any further statements for now.

Onlookers gathered on Sunday afternoon near Hoffman's apartment in a four-story red brick building in a fashionable neighborhood of the West Village, where many other actors keep homes. The entire block was cordoned off by police.

Rachel Melman, a neighbour who described herself as a fan, said she frequently saw him around the neighborhood.

"I never spoke to him, but I always wanted to," she said, adding that she would see him sitting on the scaffolding of the building, often dressed in socks and no shoes, "just reading and hanging out out there.

"Of course I'm sad. It was such a shocker," she said.

CNN, citing a law enforcement official, reported that Hoffman was last seen alive at 8 pm Saturday. He had been expected to pick up his children on Sunday but failed to show up, prompting playwright David Katz and another person to go to his apartment, where they found him dead, CNN said.

Hoffman spoke in the past of struggling with drugs, including a 2006 interview in which he told CBS he had at times abused "anything I could get my hands on. I liked it all."

Born in upstate New York near Rochester, Hoffman won the Best Actor Oscar for the 2005 biographical film "Capote," in which he played writer Truman Capote. He also received three Academy Award nominations as best supporting actor, for "The Master" in 2013, "Doubt" in 2009 and "Charlie Wilson's War" in 2008.

After more than a dozen earlier roles, Hoffman burst onto the film scene in 1997's "Boogie Nights," in which he played a lovelorn gay man in a movie about the porn industry that helped make Mark Wahlberg a star.

Hoffman, who brought a workmanlike intensity to his roles, often played characters with innate intelligence and logical minds riven by underlying passion. The blond, thickset actor's on-screen persona could range from professorial to unkempt, from the aloof intellectual to the everyman. - Reuters