ATHENS, Greece (AP) — Thousands of police have been deployed in Athens and the northern city of Thessaloniki on Thursday for commemorative marches to mark the anniversary of a 1973 student uprising that was brutally crushed by the military dictatorship then ruling Greece.

The anniversary is marked each year by marches to the U.S. Embassy in Athens, and the demonstrations have often, but not always, turned violent.

Around 5,000 police were expected to be deployed in the Greek capital, where major streets were to be blocked to traffic and three subway stations along the march route shut down on Thursday afternoon.

In 1973, the military regime that had been in power since 1967 sent police and troops to crush student-led pro-democracy protests centered in the Athens Polytechnic, a university in the center of the capital. Officers opened fire on unarmed demonstrators and bystanders, and an army tank smashed through the gates of the Polytechnic, behind which many students had gathered.

At least 20 people are believed to have been killed, although the exact death toll of the November 1973 events has never been definitively determined.

The uprising was followed by a putsch within the junta which brought even more hard-line officers into power. Democracy was restored in Greece in July 1974, after the dictatorship collapsed in the face of a Turkish invasion of Cyprus, provoked by the junta’s own machinations aiming to unite the island, whose majority population is Greek-speaking, with Greece.

Demonstrators have marched to the U.S. Embassy every year since 1974 in protest of Washington’s support at the time of the dictatorship in Greece.