ATHENS, Greece (AP) — 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 U.S. 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.