Students at Bangkok's Thammasat University had been protesting the return from exile of a hated former dictator when they were trapped by a right-wing mob and heavily armed paramilitary police, who fired guns and grenades at the defenceless crowd of several thousand.

After subduing the students, thugs rushed in and grabbed as many as a dozen. They were then taken to a nearby public field, beaten to death, hanged and otherwise abused, with the bodies unceremoniously tossed onto a makeshift funeral pyre. The official death toll was 46, though credible independent estimates put it at more than 100.

The disorder was used as an excuse for the army to seize power later that day, undoing a student-led democratic revolution of three years earlier.

This year's commemoration has drawn broader interest than usual because an invited speaker, Hong Kong democracy activist Joshua Wong, was not allowed into the country by Thai authorities, making headlines worldwide. Wong was supposed to speak at Bangkok's Chulalongkorn University, which this year for the first time was joining Thammasat in marking the anniversary.

The rector of Thammasat, Somkit Lertpaithoon, said the university teaches its students about the violence, and even has survivors on its staff.

"Even though the events of Oct. 6 may not be documented in Thai history, the new generation still strives to learn about it," he said in a speech at the university to mark the day.

The Thammasat massacre has always been a sensitive issue, both because the images of lynchings speak to a dark side of the Thai character and because the assault on the university showed how the state can carry out human rights abuse with impunity - no perpetrators were ever punished.

The latest anniversary comes as Thailand is again under military rule since a 2014 coup d'etat, and as an increasing awareness of human rights since 1976 has led to much questioning of the use of state violence, especially because of a sometimes violent struggle for political power that has troubled Thailand for the past decade, including bloody street battles in the capital Bangkok in 2010.

"The massacre is still of interest 40 years after the fact because it remains officially unresolved. Those who were involved in the violence have not been held to account, even as there has been a wave of transitional justice processes around the world, and even expanded questioning and investigations in relation to the violence of April-May 2010," Tyrell Haberkorn, a fellow in political and social change at the Australian National University, said earlier this week before the anniversary.

"The incident has relevance to the current state of Thai politics because it becomes possible to continue to stage coup after coup while repressing dissent because those who have done so in the past have not been held to account for doing so," she said.

Patporn Phoothong, a member of a research team probing the events of 1976, said one goal is to identify the people in a famous photo showing a man using a folding chair to beat a disfigured corpse hanging from a tree while a crowd watches. No one has learned the names of the victim, the attacker or any of the spectators

"The obstacle that we face, I think it is because even though 40 years have passed, people are still scared and afraid to talk and do not feel that talking will make anything better," she said.