LONDON, Oct 2, 2008 (AFP) - Amnesty International appealed to Mexico's President Felipe Calderon Thursday to establish the truth behind a deadly clampdown on student protestors 40 years ago and punish the perpetrators.

The failure to confront the massacre on October 2, 1968, when security forces opened fire on students gathered in the Tlatelolco square in the capital Mexico City, had left a 'deep scar' in Mexican society, the rights group said.

The details of that day remain unclear, even the number of those killed: international media and the US Central Intelligence Agency said about 300 people died. The then president Gustavo Diaz Ordaz put the number at between 30 and 40.

'The failure of the Mexican government to establish the truth of what happened on the night of October 2, 1968, has left a deep scar in Mexican society that can only be healed by full disclosure, bringing the perpetrators to justice and providing reparations to the victims or their families,' said Kerrie Howard, deputy director of Amnesty's Americas programme.

'President Calderon's government has been all but silent on this dark chapter in Mexico's history.

'We challenge this administration to open all relevant archives and records, establish a new and independent inquiry, and lift the obstacles preventing those responsible for this horrific crime being brought to justice.'