Heavy police presence in Athens has prevented an escalation of clashes with anarchists and other extreme leftists marking the 46th anniversary of a student uprising against the then-ruling Greek military dictatorship.

Over 20,000 people made the traditional march from the National Technical University of Athens, site of the 1973 uprising, to the US Embassy on Sunday.

Police say about 10,000 people participated in a Communist Party rally and a further 1,000 marched with the formerly ruling leftist SYRIZA party, both separate from the main march with over 10,000 participants. Former Prime Minister Alexis Tsipras started the SYRIZA march but left before the halfway point.

Police say they arrested 28 people in clashes after the march, mostly in the Exarchia neighborhood near the university, and detained a further 13.

Six people were arrested on a rooftop very close to Exarchia Square with firebombs, rocks, gas masks and gloves. Two  others were detained at an apartment house nearby, where a similar cache was found.

Police were determined this year that the scenes of, especially, 2017, as well as last year’s, when groups of rioting youth rained fire and rocks on riot police and passers-by below, would not be repeated. They seem to have succeeded so far, although they are still on alert. A standoff between police and anarchists at a corner of Exarchia Square was reported at around 8.15 p.m. but was brief, ending in a chase through the neighborhood, during which most of the arrests were made. There were reports of police heavy-handedness, including an injured reporter.

More sustained, although brief, clashes with police took place Sunday night in Thessaloniki, where groups of youths engaged in a hit-and-run with police forces, throwing firebombs and setting fire to a small number of cars in the process.