Mindst en halv million mennesker døde i den månedlange udrensning på tværs af den sydøstasiatiske øgruppe som startede efter at General Suharto beskyldte kommunisterne for et kup den 1. Oktober 1965.
Retten konkluderede at disse var forbrydelser imod menneskeheden, og sagde at USA støttede ved at tilvejebringe en liste af formodede kommunistiske partimedlemmer til Indonesien og sagde at Storbritannien og Australien gentog propagandaen fra det indonesiske militær.
Kommentar: Denne artikel er blot delvis oversat til dansk af sott.net fra: Making the world safe for psychopaths: America, Britain and Australia were complicit in Indonesian genocide in which 500,000 were killed during 1960s purge
Suharto took power on the back of the killings and then ruled Indonesia with an iron fist for three decades, during which the onslaught was presented as necessary to combat the communist threat.
Even since his 1998 downfall, successive governments have refused to apologise for the killings.
Set up by activists, the International People's Tribunal on 1965 Crimes Against Humanity in Indonesia (IPT 1965) was overseen by seven international judges in November in The Hague and its findings were read on Wednesday.
Chief judge Zak Yacoob, a South African former top justice, announcing the tribunal's findings, described a 'systematic attack against the (Indonesian Communist Party)... its affiliate organisations, its leaders, members, supporters and their families'.
As well as the killings, he said alleged communists and others suffered imprisonment, enslavement, torture and sexual violence in the tumultuous period.
The court carries no legal weight but activists hope it will pressure Jakarta to do more to come to terms with one of the worst mass killings of the 20th century.
Jakarta rejected Thursday the findings, but Yacoob urged the Indonesian government - which did not answer an invitation to attend the hearings - to issue an apology, investigate alleged crimes against humanity and compensate victims.
Foreign ministry spokesman Arrmanatha Nasir said Indonesia was under no obligation to follow the recommendations of the tribunal as they were 'not legally binding'.
Security Minister Luhut Panjaitan criticised its conclusions, saying the killings were 'none of their business, they are not our superiors and Indonesia has its own system'.
The tribunal also accused the US of being complicit in the massacres by providing lists of alleged communist party officials to the Indonesians, and said Britain and Australia had recycled the Indonesian army's propaganda.
A spokesman for Australia's foreign affairs ministry said the court was 'not a formal international court or tribunal, but a human rights NGO' and that Canberra rejected 'any suggestion that it was complicit in any way in those events of 50 years ago'.
The British and American embassies in Jakarta did not immediately comment.
President Joko Widodo, seen as a break from a string of rulers with roots in the authoritarian past, has backed public discussions about the killings, but has also refused to apologise on behalf of the state for them.
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