Thousands of people demonstrated on Thursday in Athens in a yearly protest that marks the anniversary of a deadly crackdown on a 1973 student revolt against a US-backed junta. 

Some 5,500 people marched in the capital under the supervision of security forces, as violence often breaks out on the sidelines, the police said.

Athens deployed 5,700 officers, who were backed by drones, a helicopter, and water cannon, the police said earlier.

The annual protests mark the day when at least 24 people were killed at the Athens Polytechnic when the junta sent troops and police forces against a pro-democracy student uprising.

"Two (young boys) died in my hands," Melpo Lekatsa, who was helping dress wounds at the Polytechnic the night of November 17, 1973, told state TV ERT on Thursday.

The brutal crackdown was a shock to Europe and is considered to have broken the dictatorship's grip on power, leading months later to the restoration of democracy.

"It was a heroic act by people who moments earlier, hadn't realised that they would be unafraid of bullets and who would place their bodies in front of tanks," Lekatsa, who was arrested and tortured by the junta, said.

The Polytechnic "made people realise the junta was more brutal than they had imagined," she added.

Last year's demonstration saw around 20,000 people taking part in Athens, with another 14,000 in the second city Thessaloniki.

The unrest mostly takes place in Exarcheia, the bohemian Athens district that is a popular anarchist hideout.

Since August, scores of police forces have been deployed in Exarcheia to guard regeneration projects including a controversial new metro station.

On Thursday, Prime Minister Kyriakos Mitsotakis, whose government is embroiled in a "wire-tapping scandal", said the uprising "established the most complete democracy our country has ever known."

Each year, the demonstrators hold the bloodstained Greek flag that flew that night over the Polytechnic's iron gate, which was crushed by a tank.

The demonstrations culminate at the US embassy to protest Washington's support for the Greek military dictatorship during the Cold War.

"Past demonstrations have turned violent and have involved destruction of property," the US embassy in Greece said in a statement.

"The embassy has advised its personnel to avoid the areas of the demonstrations and will close early," it said.