Fake news about Indonesia's omnipresent bogeyman - communism - has been blamed for a riot in Central Jakarta that injured five police officers and damaged vehicles in the early hours of Monday morning.
Police were forced to fire tear gas and water cannons to disperse anti-communist protesters who began to pelt police with water bottles and stones and attempted to force their way into the offices of the Jakarta Legal Aid Institute.
A weekend seminar on the 1965 anti-communist purge - a dark chapter in Indonesia's history that remains extremely sensitive today - had already been banned by police on the grounds the organisers had not applied for a permit.
But this did not stop crowds chanting "Crush the PKI" (the now defunct Indonesian communist party) and surrounding the institute building.
The Indonesian Legal Aid Institute claimed "clearly hoaxes or false news have been broadcast ... with instructions for attacking (the institute) done systematically and extensively".
It asserted false claims included that the planned historical seminar was a re-emergence of the PKI and participants intended to sing genjer-genjer, one of the most controversial songs in Indonesia.
Genjer-genjer, which was adopted as a protest song by the PKI, was banned under the Suharto regime, amid military claims that female communists had tortured six generals while singing the song.
"People said we are PKI - that's the hoax," Muhammad Isnur from the Indonesian Legal Aid Institute told Fairfax Media.
"They said PKI was holding an event. It's not true. We wanted to hold an academic discussion about what happened in 1965."
Police have detained 37 people suspected of involvement in the riot.
Jakarta police spokesman Argo Yuwono told Fairfax Media that police had informed the institute that the planned seminar could not go ahead because the organisers did not have a permit.
But Mr Isnur said the police were "just making it up". "Why would we need a permit for an internal, closed door discussion in our own office? We hold discussions every day."
The 1965 tragedy was triggered by the kidnapping and murder of several high-ranking army officers, which was blamed on the PKI.
Last month Indonesian authorities disbanded a workshop in East Java on the findings of an international tribunal into the 1965 massacre - also on the grounds organisers didn't have a permit.
In 2015 the Ubud Writers Festival cancelled sessions discussing 1965 - the first act of censorship in the history of the popular international event.
Amnesty International issued a statement last month saying there had been at least 39 cases since 2015 where authorities disbanded events related to 1965.
"These actions are a clear violation of the rights to freedom of expression and peaceful assembly," Amnesty said.
Asked if hoax news had inflamed tensions at the weekend, Mr Argo said: "Listen, if people get together to make speeches, discussion, dialogue, they must notify the police, this should be understood by people who work in the legal business."
Fake news was a huge problem in Indonesia in the lead-up to the gubernatorial election in February, with much of it targeting the ethnicity of former Jakarta governor Basuki "Ahok" Tjahaja Purnama.
Hoax news included that Indonesia was being flooded by 10 million Chinese workers, that its new currency bore an image of the banned communist hammer and sickle, that Ahok's free Human Papillomavirus vaccine program could make girls infertile and that China was waging biological warfare against Indonesia with contaminated chilli seeds.
Smear campaigns during the last presidential election also asserted President Joko Widodo was a Christian and communist.
"Don't forget, negative (news), slander, reproaching each other, hoax and fake news are spreading in social media today. They also become our challenge in the future," President Jokowi told a group of boys scouts in Central Java on Monday.
Last month police arrested three people accused of spreading hoaxes against President Jokowi and Ahok, among others, on a "news" website known as saracen, which allegedly charges clients to publish and spread fake news.
"There is clearly a growing industry around the production of disinformation (false information spread to deliberately deceive) in Indonesia and elsewhere around the world," says Australian National University academic Ross Tapsell, an expert on social media in Indonesia.
"Of course, Indonesia has a long history of government and non-government anti-PKI propaganda designed to incite and enrage," he said.
"So the material may not have changed, but the technology used to disseminate it is changing rapidly."
