Riot police used teargas and pepper spray to push back protesters, some of whom hurled rocks and Molotov cocktails at them. The demonstrators, including hundreds of students, left-wing activists and self-styled anarchists, set fire to rubbish bins and bus stops in and around the central Syntagma Square as well as in Exarchia district.

Organizers claimed that around 10,000 people had taken part in the Athens demonstration, while police put the figure at about 5,000.

At least 21 people were arrested, according to police. Violence was also reported in the northern city of Thessaloniki where similar demonstrations were also held on Saturday.

The marches were organied to commemorate the fatal police shooting of 15-year-old teenager Alexandros Grigoropoulos in Athens in 2008, which led to the worst rioting the country had witnessed in decades.

The anniversary of the teenager's death draws thousands of anti-establishment protesters to the streets each year, but this year the demonstrations were also held in solidarity with one of Grigoropoulos' friends, Nikos Romanos, who was standing beside him when he was shot dead.

Romanos, 21, who is serving a jail term for attempted bank robbery, has been on a hunger strike since last month, and doctors say his health is deteriorating. He is demanding to be allowed to attend university classes.

Anti-government protests have been frequent in Greece in recent years, as trade unions and left-wing parties mobilize their membership to voice their disapproval of austerity measures introduced in return for European Union and International Monetary Fund financial bailouts.

shs/pfd (AP, Reuters, dpa)