Greek Prime Minister Georgios Papandreou on Sunday called for a referendum in the fall on constitutional reform.
Speaking in parliament, Papandreou said the referendum should discuss "changes to the political system" but gave no details.
Greece is on the verge of defaulting on a 350 billion euro government debt despite a 100 billion euro bailout loan from the European Union and the IMF.
Papandreou's center-left government has approved crucial budget cuts in return for international loans. The move has trigged large-scale protests in the country, prompting Papandreou to replace the finance minister along with other government officials.
