Police say about 50 protesters have been detained during hour-long clashes outside Parliament.
The violence involved about 200 youths who hurled firebombs and rocks at riot police, and smashed office windows and set fire to trash bins.
Many of the mask-wearing protesters carried wooden bats and pieces of smashed paving stones, in the worst clashes seen since Prime Minister Alexis Tsipras's left-wing government was formed six months ago.
The clashes died down as a debate got underway in parliament on a new austerity bill.
Earlier, more than 10,000 people, supporters of left-wing groups and a Communist-backed trade union, staged a peaceful rally in central Athens.