Thousands of Greeks marched behind a blood-stained national flag on Saturday to commemorate the violent suppression of a 1973 student uprising against a US-backed military junta.

The bloodied flag that flew over the Athens Polytechnic on the night of November 17, 1973 -- seen as a key moment in the restoration of democracy to Greece -- was carried as usual at the head of the annual demonstration in memory of those who died.

Student associations, members of the main opposition party, the radical left Syriza, and other leftist groups were among those who took part in the march through Athens.

Police said about 20,000 people marched in total in the capital.

"The message of the Polytechnic is more current then ever," said a 28-year-old jobless man, Sotiris Lambrou, in Athens.

Lawmakers recently voted a new round of painful austerity measures that have caused widespread discontent amid the country's ongoing economic crisis.

"Only now the tanks have been replaced by the memorandums," Lambrou said, referring to the agreements between the heavily indebted country and its EU-IMF creditors.

"I think the situation now is worse than back then... back then people were not hungry," added a pensioner who gave her name only as Georgia.

The Communist Party also held its own demonstration.

Shouting chants about the economic crisis such as "write off the debt" and "capitalists should pay for the crisis", protesters walked towards the US embassy, the traditional culmination of the demonstration.

Outside the embassy, some of the protesters set fire to an American flag.

Security forces blocked the road to prevent demonstrators continuing on to the Israeli embassy to protest against recent air raids in the Gaza strip.

The road was later opened and protesters walked to the embassy where some set fire to an Israeli flag.

Nearly 7,000 police were deployed from early on Saturday to prevent any violence and only minor clashes were reported.

In the northern city of Thessaloniki, about 12,000 people marched in commemoration of the uprising, with some minor clashes also reported according to police.

In the western city of Patras, police responded with tear gas after a group of young people damaged a bank and parked cars, according to the Athens News Agency.