Dozens of hooded men early Sunday firebombed the premises of a Greek subsidiary of a Canadian gold mining company opposed by environmentalists, injuring a guard and damaging equipment, police said.
The group of 40 to 50 men threw molotov cocktails and set fire to equipment at the Hellenic Gold site in the northern region of Halkidiki at around 1:00 am local time, damaging containers and trucks, a police source told AFP.
The site has faced opposition from citizens' groups who fear the project will cause irreversible harm to the local environment.
One of the guards was taken to hospital with respiratory problems and light injuries, while 27 people were initially detained and later released.
Later in the day, police reportedly detained another four persons and were in the process of carrying out DNA tests based on evidence found on the scene, such as unused mototov cocktails, bottles, a hood, cigarette butts and blood drops.
State broadcaster Net said police are seeking another four persons.
The company released a statement appealing to the authorities to defend legality and protect efforts to invest in Greece.
"All of the company's activities in the region take place in accordance with legal procedures... the measures taken to protect natural environment more than cover the terms set by Greek and European law," the statement said.
The citizens' groups have been trying to halt the project since 2011, when the Greek government allowed Hellenic Gold, a subsidiary of Canadian company Eldorado Gold, to dig in the region.
Another Canadian company, TVX, began an operation in Halkidiki nearly two decades ago before pulling out in 2003.
The protesters are backed by the radical leftwing opposition party Syriza, now the second largest group in parliament.
In January, hundreds of people demonstrated in Athens, with some carrying felled oak and beech trunks on biers and wearing decontamination suits.
The mayor of Thessaloniki and local authorities support the new project, which is expected to create hundreds of jobs in the recession-hit country, whose unemployment rate has topped 26 percent.
Critics say the project will not only drain and contaminate local water reserves but also fill the air with hazardous chemicals including lead, cadmium, arsenic and mercury.
A picturesque and forested peninsula, Halkidiki is a popular tourist destination, particularly for Russia and the neighbouring Balkan states.