Iceland's overextended financial system collapsed last month and the country's currency, the krona, has lost half its value since January.
The government recently took over the country's biggest banks to prevent them from collapsing.
Together, they owe more than £40 billion, about six times the value of Iceland's annual economic output.
Reykjavik accepted a combined $2.1 billion (£1.4bn) loan from the IMF last week, making Iceland the first western European country to be "rescued" by the US-dominated body since Britain in 1976.
One protester climbed onto the balcony of the parliament building in Reykjavik and hung up a banner reading: "Iceland for Sale - $2.1 billion."
Others held up a banner reading: "The IMF will crush education, welfare, health care and democracy."
One of the speakers at the rally, law student Katrin Oddsdottir, warned the government that people would storm the parliament and other government buildings if it failed to call new elections within a week.
"Peaceful protest will do in peacetime, but there has been an attack on the Icelandic constitution and democracy in Iceland," Ms Oddsdottir said.
"We will carry you out!" she stormed, adding: "The Icelandic people will not be subjugated."
After the rally, protesters broke into a police station to demand the release of a demonstrator who was detained on Friday.
Police used pepper spray in an attempt to prevent them from entering the station, but at least 20 people managed to get in.
The confrontation ended when police freed the man after someone reportedly paid his outstanding fine from a previous demonstration.
When Mr Oddsson was prime minister from 1991 to 2004, he pushed through a neoliberal programme of privatisation, tax cuts, reductions in spending and deficits, central bank independence and exchange rate flexibility.
His government privatised Iceland's top banks at the beginning of the decade and slashed corporate tax rates from 50 per cent to 18 per cent.
The current government was elected last year and does not need to call elections until 2011.
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