Charlie Sheen: I'm HIV Positive and I've Been Blackmailed for Millions Over It

Charlie Sheen said it directly, "I'm here to admit I'm in fact HIV positive" -- and defiantly stated he would no longer fork over millions of dollars to buy silence from friends and sexual partners.

Sheen spoke calmly about his diagnosis with Matt Lauer on "Today"... saying he needed to "put a stop to this onslaught of attacks and untruths." He said he's known for roughly 4 years that he is HIV positive, and had "told enough people that I've trusted to be in the position that I'm in today."

TMZ broke the story ... Charlie says he was betrayed by people he trusted with knowledge of his diagnosis, but they ended up revealing it. Lauer asked if Sheen was still paying people, and he said ... "Not after today I'm not! I release myself from this prison today."

He also talked about the moment he found out about the diagnosis, saying he initially thought he had a brain tumor. As for infecting others ... Sheen said he's warned all of his sexual partners ahead of time -- and there were only 2 with whom he'd had unprotected sex, but they "were under the care of my doctor."

As we've reported ... our sources say Charlie has paid several settlements to sexual partners who said they were NOT warned ahead of time.

Sheen says he doesn't know how he contracted the virus, but feels he has "a responsibility now to better myself and to help a lot of other people."