Prime Minister George Papandreou of Greece has vowed tough action to save the country's finances.
''We will push through all the major changes our country has needed for years," Mr Papandreou said in a speech at a trade fair in Thessaloniki.
Clashes between police and protesters in Thessaloniki resulted in more than 100 people being detained. At least 20,000 people joined the protests.
Greece is struggling to meet the terms of an international loan package.
In July, eurozone leaders announced a bailout worth 109 billion euros ($US155 billion), in addition to a 110 billion euro lifeline granted last year.
The BBC reports Mr Papandreou's speech was designed to reassure EU and IMF lenders that Greece would press ahead with unpopular policies.
''We decided to fight the battle to avoid a disaster for the country and its people and to stay in the euro,'' he said.
''Any delay and wavering is dangerous for the country.
''We don't have the right to abandon this effort halfway through because if it remains half-done, (our) sacrifices will have been in vain,'' he added.