Charlie Sheen, former star of the hit US television comedy “Two and A Half Men,” said on Tuesday he was diagnosed HIV positive some four years ago and had been extorted for more than $10 million to keep the information quiet.

Sheen, 50, told NBC's “Today” TV show he was speaking out because he was being blackmailed, and to refute tabloid reports that he has AIDS and was spreading it to other people.

“I have to put a stop to this onslaught, this barrage of attacks and of subtruths and very harmful and mercurial stories that are about me, that threaten the health of so many others that couldn't be further from the truth,” he said.

“I am here to admit that I am in fact HIV positive,” Sheen said, adding he was “not entirely sure” how he contracted the virus.

“It started with what I thought was a series of crushing headaches,” he said. “I thought I had a brain tumor. I thought it was over.”

Sheen's doctor, Robert Huizenga, also appeared on the “Today” show and said the actor “does not have AIDS.”

Sheen, who is three times divorced, played the womanizing bachelor Charlie Harper on top-rated comedy series “Two and A Half Men” for eight years before being fired in 2011 for bad behavior that included cocaine-fueled partying and a conviction for assaulting his ex-wife.

At the time, he was the highest paid actor on US television, with a reported salary of some $1.8 million per episode.

Asked whether he had transmitted HIV to anyone since his diagnosis, Sheen said on Tuesday, “impossible.” He said he had informed his ex-wives immediately after getting his diagnosis.

Sheen acknowledged he had paid people “upwards of $10 million” in recent years for their silence about his condition but said he would no longer do so.

“I think I released myself from this prison today,” he said.

Sheen, the son of “West Wing” TV actor Martin Sheen, said he doesn't feel any stigma from being HIV positive, a condition that affects more than 1.2 million people in the United States alone, according to government statistics.

HIV, or human immunodeficiency virus, weakens the immune system by destroying cells that fight disease and infection. No effective cure exists, but with proper medical care it can be controlled. If not, one can be infected with AIDS.

“I have a responsibility now to better myself and help a lot of other people. Others may come forward and say thanks Charlie. Thanks for kicking the door open,'” he said.

Reaction to Sheen's announcement was largely compassionate on social media where the actor won sympathy for disclosing his condition.

Sheen also starred in 1980s movies “Platoon” and “Wall Street,” as well as the 2012-2014 cable TV comedy “Anger Management,” which was closely based on his life.

He has five children from his marriages to model Donna Peele, actresses Denise Richards and Brooke Mueller, and other relationships.