Thousands of Greeks marked on Saturday the 39th anniversary of a student uprising which led to the restoration of democracy in Greece, with a customary mass march in central Athens, chanting anti-austerity slogans.

Political party leaders, academics, youth and elderly ordinary citizens flooded the courtyard of the National Technical University of Athens, where the pro-democracy struggle culminated, to place flowers at the monument honoring more than 20 students and civilians killed.

In their messages for the day, Greek President Karolos Papoulias and other state officials stressed that the commemoration of the uprising in difficult times for Greece, as the country faces a severe debt crisis since 2009, underlines the "need for Greeks to continue efforts to overcome recession and strengthen social justice."

Starting from the university site students had taken over in 1973 calling for "Bread, Education and Freedom," some 20,000 Greek citizens, according to estimates, participated on Saturday afternoon in a rally organized by student unions, chanting similar slogans.

The march traditionally ends each year in front of the US embassy in Athens in protest of Washington's support for the junta at the time.

Demonstrators set American flags on fire, as a group of Greek Communist Party members was to continue this year to the Israeli embassy in protest of this week's Israeli strikes against the Gaza Strip.

More than 5,000 policemen were deployed across the Greek capital, amid fears for violence from anarchists who have marred similar peaceful protests several times in recent years.

Saturday's rally reflected Greek people's frustration over the ongoing austerity drive launched to avoid a chaotic default which has, however, fuelled unemployment and recession.

Protesters denounced the fresh set of tough austerity measures pushed by the government coalition two weeks ago in order to ensure further aid from the European Union and the International Monetary Fund lenders who have kept the country afloat since 2010.

Without the next multi-billion-euro bailout tranche expected to be released within November, Greece could financially collapse and likely exit the euro, triggering instability across the entire eurozone, by the end of this year, Greek and foreign officials and analysts warn.