By Nick Miles BBC correspondent in Mexico City

The Mexican Government has opened millions of secret security files to shed light on human rights abuses of the past.

The files, covering cases from more than four decades up to 1985, include documents related to the torture and killing by security forces of hundreds of political activists.

President Fox campaigned promising greater openness

Dealing with Mexico's extremely patchy human rights record was one of President Vicente Fox's key pledges when he came to power 18 months ago.

Fittingly, it was the president who made the announcement at a ceremony at the National Archives Centre in the capital.

"Whatever the consequences, we will clarify what went on and what abuses were carried out," he said.

Student massacre

Hundreds of people died and up to 500 went missing during campaigns by the Mexican security forces in the late 1960s and 70s.

The government at the time, the Institutional Revolutionary Party, which ruled Mexico for seven straight decades, said they were carrying out legitimate actions against armed left-wing insurgents.

Soldiers were ordered to fire on protesters, it was claimed

But witnesses, like those who were at the shooting of up to 200 student demonstrators in Mexico City in 1968, claimed soldiers often opened fire without warning and in other instances, carried out extra-judicial killings.

It is not certain what will be done with the newly released documents.

But President Fox said his government would not shrink from prosecuting anybody, including former presidents.

He said he was after the truth and nothing more.