Demonstrators and riot police fought pitched battles in the Greek capital Athens on the seventh anniversary of a teenager's killing by police.
On Sunday, violence erupted at the end of a peaceful march through Athens after police attacked those taking part in the event.
The protesters subsequently threw scores of petrol bombs, bricks, and broken marble at police forces who responded with tear gas and stun grenades.
Minor damage was caused to a bank and some garbage bins were set on fire, according to witness accounts. There were no reports of causalities, but police detained at least 18 people.
More than 6,000 police officers were deployed across Athens.
Demonstrators from all walks of life came out and carried banners reading "Never forget,” and "Our fury will not end. Cops, pigs, murderers."
Back in 2008, the shooting of 15-year-old Alexandros Grigoropoulos, led to the country's worst riots in decades.
Since then, a huge rally is held in Athens every year, which draws thousands of anti-government and anti-austerity protesters.