Police detained 70 people during a protest against the gold mine in Skouries forest, Halkidiki, on Sunday.
Authorities said they were forced to act, using tear gas and stun grenades, after some of the protesters started throwing rocks at policemen.
Protesters accused the riot police of kettling the crowd and trapping a group of demonstrators who were forced to climb onto a bus.
The protest took place a few days after the government halted work on the gold mine, giving the company a year to review its technical study or face having its permit revoked permanently.
Hellas Gold, the firm which manages the mine on behalf of Canadian giant Eldorado, said it would take legal action against the government and might have to fire staff in the meantime.