Greek riot police and demonstrators clashed Saturday in central Athens as thousands gathered to protest the Iraq war.
Police fired tear gas into the crowd after individuals threw rocks at officers and set a squad car and a bank on fire.
There are no immediate reports of injuries.
The marchers were on their way to the U.S. embassy in the Greek capital.
Saturday's protest marks the fourth European Social Forum, which is meeting in Athens. The organization describes itself as a gathering of "civil society groups and movements opposed to neo-liberalism and a world dominated by capital."
Some information for this report was provided by AFP, AP and Reuters.