Eight people living in tents have been arrested for "degrading and polluting" the square. Among them are two Frenchmen, a German and a Romanian, police say.
Athens city crews have removed the tents, stands and banners that have filled the square since May 25, when the encampment of people protesting the government's austerity policies was set up.
Athens Mayor Giorgos Kaminis had called Friday for the removal of the encampment that, he said, had degraded the city center's image.
Calls for a protest and a "people's assembly" later Saturday have been posted in a website used mostly by extreme leftists.