Riot police on Tuesday set up a cordon around the historic building of Athens Polytechnic in downtown Exarchia after coming under attack from self-styled anarchists.
The clashes started after a group of a few dozen self-styled anarchists broke away from a May Day rally in the city center and took refuge in the university building in order to launch the attack on police.
They were throwing rocks and homemade petrol bombs, and have barricaded Stournari Street with flaming trash bins.
Police had closed Patission Street for a few hours but gave it back to motorists to facilitate the flow of traffic in the city center.
The city center had been closed for most of the morning as hundreds turned out for rallies organized outside Parliament by labor unions GSEE and ADEDY, while the communist-affiliated PAME held a march to the American Embassy.