Oscar-winning director Oliver Stone has attacked two forthcoming films about Julian Assange after revealing that he met the WikiLeaks founder at the Ecuadorian embassy in London last week.

Stone, a long-time supporter of Assange, tweeted a picture of himself with the political activist. He wrote: "A sad occasion in that Julian could not follow me out the door. He lives in a tiny room with great modesty and discipline."

In further tweets, Stone added: "Strong mind, no sun, friends who visit, work to be done, one documentary coming out from Alex Gibney that is not expected to be kind.

"Another film from Dreamworks which is also going to be unfriendly … I don't think most people in the US realise how important WikiLeaks is and why Julian's case needs support.

"Julian Assange did much for free speech and is now being victimised by the abusers of that concept."

The films criticised by Stone were Alex Gibney's forthcoming documentary We Steal Secrets, which debuted at Sundance in January, and Dreamgirls director Bill Condon's drama The Fifth Estate, which stars Sherlock's Benedict Cumberbatch as Assange alongside Laura Linney, Anthony Mackie and Peter Capaldi. It is due to be released in the US in November, which suggests an awards-season run in 2014.

Assange himself has attacked The Fifth Estate, which is based on former aide Daniel Domscheit-Berg's book Inside WikiLeaks: My Time with Julian Assange at the World's Most Dangerous Website, as well as Guardian writers David Leigh and Luke Harding's WikiLeaks: Inside Julian Assange's War on Secrecy. Speaking via video link to the Oxford Union in January, he labelled it "a massive propaganda attack" that told "lie upon lie".

Assange is living in the Ecuadorian embassy, which offered him asylum in August, and is hoping to avoid extradition to Sweden, where he faces allegations of rape and sexual assault. He says he fears being extradited from Sweden to the US over his WikiLeaks activism.