Clashes broke out between youths and police around Syntagma Square in Athens on Friday.
The violence took place just before the start of the ‘no’ rally, ahead of a referendum on Greece’s future on Sunday.
Police threw stun grenades during scuffles with a few dozen people, many dressed in black and carrying red flags, which are often carried by anti-establishment radicals.
Some attacked heavily shielded officers while police reinforcements on motorbikes were quickly mobilised.
Pepper spray was also used to stop protesters damaging property, leaving some people dazed and coughing. At least one person was arrested.