The clashes came as 20,000 people marched through the centre of the city and during a 24-hour general strike that has brought most public services to a halt, grounded flights and stopped all trains and island ferries.
Riot police clashed with gangs of black-hooded youths just yards away from where senior European Union and International Monetary Fund officials were meeting to discuss another £10.4 billion slice of aid to save Greece from bankruptcy.
Police said 17 demonstrators and two policemen were hurt in the clashes, including one protester with serious head injuries, while 24 people were detained.
Greek unions have questioned whether further austerity should be imposed on the country at a time when the economy badly needs growth.
This month, the Socialist government is planning to pass another £20 billion of cuts over the next four years.
While the measures have been deeply unpopular in Greece, there remains scepticism elsewhere in Europe over the package.
“Before we can talk about further aid, Greece has to make sure that all austerity and reform measures are duly implemented,” said a spokesman for the political party of Angela Merkel, the German chancellor.
Previous protests have also been marred by violence, and three clerks died last May when their bank was torched by rioters.