Greece's two governing parties, which back tough austerity measures, have lost their parliamentary majority in an election on Sunday.
With almost all votes counted, centre-right New Democracy is leading with 18.9%, down from 33.5% in 2009.
Centre-left Pasok is in third place with 13.2%, down from 43.9% in the last elections. Left-wing coalition Syriza is in second place with 16.7%, the BBC reports.
Pasok and New Democracy have been in coalition since November last year.
There is widespread anger across Greece to harsh measures imposed by the government in return for international bailouts.
Syriza opposes the government's austerity measures.
The neo-Nazi Golden Dawn party could enter parliament for the first time if their tally of almost 7% holds up.
New Democracy leader Antonis Samaras said he would form a national salvation government to keep the country in the euro.
However, Mr Samaras said he would seek to "amend" Greece's controversial bailout agreement with the European Union and the International Monetary Fund in order to boost growth.
Syriza leader Alexis Tsipras said he wanted to form a left-wing coalition rejecting the terms of Greece's bailouts.
"The parties that signed the memorandum (with the EU and the IMF) are now a minority. The public verdict has de-legitimised them.
"Our proposal is a left-wing government that, with the backing of the people will negate the memorandum and put a stop to our nation's predetermined course towards misery."
Pasok leader and former Greek Finance Minister Evangelos Venizelos has called for a broad coalition government of pro-European parties.