ATHENS, Greece (AP) — Thousands of people marched Thursday through central Athens, accompanied by a heavy police presence, to mark the anniversary of a 1973 student uprising that was brutally crushed by Greece’s military dictatorship at the time.

The anniversary is observed each year with marches from Athens Polytechnic university to the U.S. Embassy and have often, though not always, turned violent. Thursday’s march was peaceful.

However, clashes broke out at a similar march in the northern city of Thessaloniki to the U.S. consulate, with a small group of demonstrators throwing Molotov cocktails and rocks at riot police, who responded with tear gas and stun grenades.

In Athens, the demonstration was led, as it is each year, by a group carrying a blood-stained Greek flag from the 1973 uprising to the embassy to protest Washington’s support of the dictatorship in Greece at the time.

Large crowds followed, while riot police stood by, guarding official buildings and other sites, and a police helicopter flew overhead.

In 1973, the military regime in power in Greece since 1967 sent police and troops to crush student-led pro-democracy protests centered in the Athens Polytechnic. 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, bringing even more hard-line officers to 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 with Greece.