MEXICO CITY, Oct 2, 2008 (AFP) - Mexicans were to march through the capital Thursday from the site of a bloody student massacre in memory of scores killed there 40 years ago, in yet another call for justice.

As student movements shook the world in 1968, Mexican security forces killed between 44 and 300 protesters in Tlatelolco, 10 days ahead of the Mexican Olympic Games, and rapidly cleared up the evidence.

Forty years on, the details of the massacre remain unclear, the perpetrators are untried, and impunity in Mexico is as widespread as ever.

At the end of August, hundreds of thousands demonstrated in Mexico City and across the country against growing insecurity after a upsurge of murders and kipnappings. The demonstrations were some of the biggest in recent years.

Mexico City left-wing mayor Marcelo Ebrard on Thursday ordered the Mexican flag to be flown at half-mast in municipal sites across the city and paid tribute to the 1968 student movement in a speech in the Square of Three Cultures where the massacre took place.

In contrast, flags flew at the top of their poles on buildings of the President Felipe Calderon's conservative federal government and in the capital's massive Zocalo square where demonstrations were to end up, due to a lack of official recognition of the massacre.

Amnesty International appealed to Calderon on Thursday to finally establish the truth and said his failure to confront the clampdown on some 8,000 students and workers on October 2, 1968, had left a 'deep scar' in Mexican society.

Then president Gustavo Diaz Ordaz said there had been between 30 and 40 deaths, while international media and the CIA registered some 300 killed.

The only official to have been blamed for the massacre is Luis Echeverria Alvarez, a former president who was interior minister at the time.

But he has never been punished for involvement in the event.

A group of former student leaders, Committee 68, on Thursday said it still sought to try Echeverria for genocide.

The Supreme Court of Justice ruled in 2005 that there was not enough evidence for a genocide trial against Echeverria, currently under house arrest for crimes committed during the country's 1970s 'dirty war.'

Students at the National Autonomous University of Mexico (UNAM), the heart of the 1968 protests, held a minute's silence Thursday.

Four decades on, 'there are many unresolved issues and I would signal the need to stress the courage and importance of the movement,' UNAM rector Jose Narro said.