Anti-riot officers used tear gas to contain the crowds as Prime Minister George Papandreou prepared to address the nation on the economic crisis from the city.
Barring the approach to the congress hall where Papandreou was scheduled to speak at 1730 GMT, police attempted to push back about 3,000 taxi owners picketing over changes to the industry.
The protest was kicked off earlier in the day by the group calling themselves "the indignants" after Spain's youth protest movement.
A huge banner read "We owe nothing, we pay nothing, we sell nothing, we fear nothing."
Members of Greece's private and public sector unions were also out in force, along with supporters of the local Heraklis football club protesting against their exclusion from the first division.
A record 7,000 police were on duty to prevent a predicted 20,000 demonstrators from surrounding the congress hall. They made two arrests after carrying out control checks on about 60 people.
Eurozone leaders announced a 159-billion-euro ($223-billion) rescue package for debt-ridden Greece in July, but many Greeks fear the stringent conditions set for the money to trickle down will only make unemployment worse. (AFP)
