About 40 hooded anti-establishment protesters threw petrol bombs and stones at riot police in central Athens on Saturday, setting fire to a trolley bus.
Police fired teargas to try and control the crowd outside Athens Polytechnic University, near the central Exarchia district, a Bohemian quarter popular with artists and left-wing intellectuals.
Local media said the clash started when the crowd forced the bus to stop and told passengers to get off. Their reasons were not immediately clear.
Earlier on Saturday about 200 leftist and anti-establishment protesters marched to the German embassy in Athens, in a peaceful protest against Berlin's hard stance in Greece's talks with lenders on a cash-for-reforms deal.
Last month anti-establishment protesters occupied several public buildings and briefly entered the grounds of parliament, calling for the release of jailed members of a Marxist group.