JAKARTA, Indonesia — The armed mob set out after nightfall, looking to break up — or worse — an illegal gathering of communist sympathizers in an upscale neighborhood of Jakarta, the Indonesian capital.

But when the mob arrived, its members were told that the event, held on Sept. 17 at the offices of a prominent Indonesian legal assistance foundation, was just an art show. They either did not believe that, or they did not care.

For the next few hours, the mob — a mix of several hundred hard-line Islamists, nationalist militia members and hired local street thugs, armed with rocks and sticks — laid siege to the building, smashing windows with rocks and bricks and making death threats to those inside. Police officers had to disperse the mob with warning shots and tear gas. Those attending the event, including many human rights activists, had to be evacuated to safety.

The attack was one of many examples of a sudden resurgence of anti-communist hysteria in Indonesia ahead of Saturday’s anniversary of the beginning of what historians call one of the worst mass atrocities of the 20th century: the state-sponsored purges of those suspected of being communists or their sympathizers in 1965-66.