JAKARTA (REUTERS) - Organisers of Indonesia's biggest annual literary festival said on Friday they had been forced to cancel several events centred on the mass killings of alleged communists in the mid-1960s, after authorities threatened to revoke the festival's licence.

The cancellations come as Indonesian authorities seek to avoid controversy as the country this year marks the 50th anniversary of one of the worst massacres since World War II.

"The events related to 1965 were flagged by the authorities as something that could potentially cause our operating licence to be revoked," said a spokesman for the festival, who asked not to be named due to the sensitivity of the issue.

Organisers said in a statement the cancellations marked the first time panel sessions had come "under scrutiny from local authorities - which include government, police and military officials".

Indonesia's coordinating security minister Luhut Pandjaitan declined to comment on the issue, while palace officials and authorities in Bali were not immediately available for comment.

The Ubud Readers and Writers Festival has in recent years gained significance in global literary circles, attracting award-winning authors and thousands of participants. The festival is due to get under way next week in the resort island of Bali.

The cancelled events - ranging from panel discussions to a movie screening - were related to one of the darkest periods in Indonesian history.

At least 500,000 people are thought to have died in the rampaging violence that started in late 1965 after then-general Suharto and the military took power following an abortive communist coup.

A million or more people were jailed, suspected of being communists.

Triggered in the midst of the Cold War, when the West feared that communism was sweeping through decolonising Asia, much of the slaughter was in the populous main island of Java and Bali.

Rights groups and democracy activists have called on successive governments to initiate reconciliation efforts with victims' families.

Many had pinned their hopes on President Joko Widodo addressing past human rights cases when he took office a year ago.

Widodo has declined to issue an official apology for the killings.

Organisers said it was unlikely that all discussion on the topic would be censored during the festival.

"It's not as though underlying messages about human rights and reconciliation will not be talked about at the festival," the spokesman said. "This will probably prompt more discussion."