A group of more than 20 self-styled anti-state activists, mostly younger adults, briefly took over the Hellenic-American Union's (HAU) building in downtown Athens on Monday morning.
The group unfurled a banner in solidarity with what it charged was "slavery" in US prisons.
Police later detained most of the protesters, according to reports.
The HAU organizes various educational courses and cultural programs at its multistory building.
