Police fired tear gas at hundreds of demonstrators, after some tried to break through a police cordon guarding the Greek finance ministry.
The outbreaks came after Greece began talks over extra budget cuts as conditions for the bailout loans.
These cuts would be in addition to an already mooted austerity drive aimed at reducing the nation's public deficit, which is more than four times bigger than the EU limit.
Union officials say the IMF wants Athens to raise sales taxes, scrap bonuses amounting to two extra months of pay in the public sector and accept a three-year pay freeze.
The union officials also claim that by next year, the IMF and the EU want Greece to shed 10 percentage points from the public deficit that reached 13.6% of output in 2009.
In addition, they say Athens has been asked to get rid of 13th and 14th month bonuses for public sector workers and pensioners.
It was also reported in the Financial Times in London on Friday that another measure would include raising the retirement age from an average of 53 to 67.
"Have you understood that these measures that are being recommended to you are measures of destruction?" the head of the Left Coalition Syriza, a small left-wing party, Alexis Tsipras, told Greek Prime Minister George Papandreou and fellow lawmakers.
"There is no other choice, ask for the verdict of the Greek people, call a referendum," he added.
We are holding tough negotiations to protect what we can for the weak and the middle class in our country
Greek Prime Minister George Papandreou
In the face of mounting opposition to the budget cuts, Mr Papandreou insisted that the poor must not suffer disproportionately because of the austerity drive.
"We are holding tough negotiations to protect what we can for the weak and the middle class in our country," he said.
He said spending on healthcare would continue, but that corruption in the sector would be tackled.
Negotiations about the terms of the Greek loan come as the country prepares for annual 1 May celebrations, with three demonstrations expected against the measures.
Meanwhile, there were also reports of further clashes outside the parliament building in Athens on Thursday night.
The latest protests followed a meeting between Prime Minister George Papandreou and trade union leaders, who reacted angrily to his planned austerity measures.
"We got a flavour of a very harsh package of measures, measures that will lead to recession," Yiannis Panagopoulos, head of the powerful GSEE umbrella trade union, was quoted by AP news agency as saying.
The unions have now called a general strike for 5 May.
The BBC's Gavin Hewitt in Athens says the mood is increasingly against any bailout and the Greek prime minister has said the country is in a battle for survival.
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