A final report on "war crimes" committed in Indonesia decades ago has called on the government to investigate and prosecute all those involved in the deaths of at least 400,000 of its own citizens.

The international people's tribunal formed at the Hague said on Wednesday the Indonesian state was "responsible" for the "genocide" against members and supporters of the country's Communist party and followers of President Soekarno during the 1965-66 killings.

Zak Yacoob, chief justice of the civil society tribunal, said the government of Indonesia needed to act "urgently and without qualification" to address the injustices committed in the deadly purge.

In an interview with Al Jazeera, Yacoob also said that aside from prosecuting those involved in the killings, the government must also "apologise" to all victims or their survivors, as well "ensure appropriate compensation and reparation".

Starting on October 2, 1965, Indonesia's General Suharto launched a campaign to wipe out Communist party officials and sympathisers, following a failed coup which resulted in the death of six generals.

The report also detailed 10 "gross human rights violations" carried out against civilians, including imprisonment, torture and sexual violence.

The people's tribunal was formed last year after efforts to bring the mass killers to justice had failed.

Al Jazeera's Step Vaessen, reporting from Java, where many of the deaths happened, said the outcome of the investigation provided "moral support" to the victims and their families.

Revisiting an Indonesian massacre 50 years on

"It's a very important verdict for a case that's been haunting Indonesia for the past 51 years," she said. "The question now is, what's the Indonesian government going to do with these findings?"

So far, the government has completely rejected the existence of the people's tribunal.

With no legal basis for its formation, the findings of the tribunal would also have no legal consequence, our correspondent reported.

Damien Kingsbury, an expert on Indonesia at Melbourne's Deakin University in Australia, said that while the report had no "pracitical effect" it was very important from a "symbolic point of view".

Kingsbury said that it could put pressure on the government to take more action on the issue.