Most will think of the black power salutes given by American athletes John Carlos and Tommie Smith on the medal podium in Mexico City's Olympic Stadium. 

Yet, in the build up to 1968 Games, there had also been widespread protests by students in Mexico City.

These came to a head in a mass demonstration at the beginning of October. It lasted only a few hours but when it was over, independent witnesses calculated that at least 300 had been killed and some estimated that the death toll was much higher.

The Mexican Government always denied that they had instigated the violence and instead suggested it had been the work of "outside agitators".

Their actions were not condemned at the time by the International Olympic Committee (IOC), and many files in Mexico remained closed for reasons of "national security". It means no-one has even been prosecuted for the outrage.

It had been a year of unrest throughout the world. In America, protests against the Vietnam War and in support of civil rights continued. In Paris, students had taken to the streets.

Behind the iron curtain in Czechoslovakia, a popular revolt against the Communist Government was dubbed the "Prague Spring". Led by Alexander Dubcek, a new Government promised "Socialism with a human face". 

The Kremlin's reaction to this was to send in the tanks and Warsaw Pact forces crushed the revolt in an echo of what had happened in Hungary after the uprising in 1956.

In a recently declassified telegram, the American Central Intelligence Agency (CIA) suggested that Soviet agents were behind the protests in Mexico in what they called a "subversive manouevre" designed to deflect attention from what was going on in Czechoslovakia.

In Mexico, students protested at their campuses and on the streets. These became violent. It is said that the demonstrations and subsequent military occupation of many campuses kept around 50,000 away from their studies that summer.

Although many interpreted the protests as a follow-up of those in Europe, there were additional local elements. There had been disturbances at a football match between two rival colleges which prompted a particularly brutal response from police.

Many also felt uneasy that so much was being spent on mega sporting events when so many of the country's population still lived in extreme poverty.

Mexico was not only host city for the 1968 Olympics as the nation had also been selected by FIFA to stage the 1970 World Cup finals.

The Mexican Government of the time was led by Gustavo Diaz Ordaz, an authoritarian President who was determined that nothing should get in the way of the Games. The facilities for the sports events were indeed impressive. A cultural Olympiad on a scale not seen before was also planned.

In July, CIA official William G. Bowder noted: "What does worry Mexican officials is the image projected by the disturbances and the impact on the Olympics in which they have so heavily invested."

Another CIA memo described the Mexican security forces as "tough, politically reliable and reasonably competent".

At the beginning of October, as competitors were starting to arrive in Mexico, a demonstration was announced at the Plaza de Tres Culturas or "Square of the Three Cultures" in the Tlatelolco district of Mexico City. 

It was so named because of its proximity to Aztec ruins, a colonial church and a modern ministry building.

IOC President Avery Brundage later mentioned at the opening of the full IOC Session, in a rather insensitive eulogy to President Ordaz, the support he had given to the cultural festival at Mexico City 1968.

"Mexico, a country with three distinct and outstanding cultures as illustrated in the Plaza of that name," he said.

A meeting was planned to start at around five o'clock in the evening.

It had been called to seek the withdrawal of Government troops from a University campus where protests had been ongoing throughout the summer.

Some estimated the number of students present as 5,000, others put it as double that number.The film of the event gives the impression of a peaceful protest.

The atmosphere changed when troops with fixed bayonets and armoured vehicles moved into the square. Helicopters circled above. Flares were dropped which was the signal for the assault to begin. 

Snipers in the buildings around opened fire. There are claims that the Government forces deliberately shot one of their soldiers, so as to claim that the violence had been started by the students.

Sports reporter John Rodda later described the whole experience as like the final scene of "Butch Cassidy and the Sundance Kid".

It was said that a special "Olympia" force had been established to deal with any disturbances that might disrupt the Olympics. They wore one white glove as a distinguishing mark.

In the wake of the violence, forces went from house to house to try and find fugitives from the square.

American newspapers also reported that soldiers had looted shops.

The IOC Executive Board met in the days which followed the massacre.  

Vice president Lord Killanin recalled that "the only person who did not appear to be at all upset by what had happened was the former head of the Organising Committee General Jose de Jesus Clark"

Clark insisted that "more people were killed in traffic accidents in Mexico every day than had been shot in the square that night so what was all the fuss about?"

Brundage insisted in a statement that the Games "will proceed as scheduled". 

"None of the violence has at any time been directed against the Olympic Games," he said. "We have been assured by the Mexican authorities that nothing will affect the peaceful entrance of the flame into the Stadium on October 12. 

"We have full confidence that the Mexican people known for their great sportsmanship and hospitality will join participants and spectators in celebrating the Games, a veritable oasis in a troubled world."

Rodd had spoken to the Marquess of Exeter who was then both International Association of Athletics Federations President and an IOC Executive Board member.

"We were given all the evidence by the Mexican Government," Exeter told Rodda.

In his eyewitness accounts for The Guardian newspaper, Rodda took them to task for not consulting foreign reporters who had actually been there.

For its part, the CIA described the violence as reaching "a level that raises doubt about the Mexican Government's capability to keep the Olympic events and the many foreign visitors insulated from its domestic crisis". 

They suggested that the protests "now appear determined to force the cancellation of the Olympics which they recognise as of the utmost importance to the Government".

David Hemery, a 400 metres hurdles champion from Great Britain, told the BBC: "A student came to the edge of the compound area, a wire fence. He was trying to say this not personal, this has nothing to do with the Olympics. 

"We respect what you are here for, the best in the world of sport, but the world's press are here and we have no better time to challenge the regime and the suppression we are under. Please let the people inside know."

John Coker, a boxer from Sierra Leone, told the New York Times: "We just are not in touch with what's going on between students and the police. If I hadn't gone down to get my mail today and talked to the girl at the mail desk, I wouldn't have known what happened last night."

Any visitors to the square in the following days would have been hard pressed to locate evidence of the disturbances. A clean up squad had moved in to take away the evidence.

When the Olympic flame arrived at the ancient temple grounds of Teotihuacan, the route to the historic site was lined with armed personnel. It was scarcely the image for an event which was supposed to promote peace and friendship.

There had been some concern that there would be protests against Mexican President Gustavo Díaz Ordaz Bolaños at the Games but he arrived early for the Opening Ceremony and there were no further student protests. 

"The battle in the square had squeezed the very life out of the spirit of the students," suggested Lord Killanin later.

A joint statement by the IOC and National Olympic Committees appealed to Governments "for the application of the sacred truce and hope that the Games of the XIXth Olympiad may contribute towards stopping all conflicts and towards the victory of peace and of universal fraternity and send a brotherly message of peace and friendship to all peoples".

It made no mention of the slaughter in the square.

Mexican filmmaker Leobardo Lopez Aretche's film El Grito did document the protest. Screening of the film was banned and it was only shown in secret throughout the 1970s and 1980s.

The Mexican Government remained in denial. Their original statements claimed only four casualties, figures which were immediately discredited by reporters on the ground. The number did ultimately increase but the officially accepted figure bore no relation to those reported in the overseas press.

Luis Echeverria had been a secretary of the interior at the time of the massacre and became successor to Ordaz as President. He denied giving the order to fire. He later told congressional investigators: "There was a hierarchy. The army is obliged to respond to only one man" and insisted that Ordaz was the only person who could have given the order.

Ordaz died in 1979 and no-one has ever been brought to account for the massacre.

It even took another quarter of a century before a memorial appeared in the square. Even this only displays the names of very few "Companeros Caidos" or fallen comrades.

When Mexico City was first awarded the Olympics, such was the concern over the high altitude, there had been those who warned "people will die".

The forecast came true but in a way few could have imagined.