Alexis Tsipras, the Greek prime minister, has accused the Eurogroup and the ECB of an "attempt to blackmail the will of the Greek people and to hinder democratic processes".
In a televised speech given yesterday evening, Tsipras promised that this weekend's referendum on whether Greece should accept austerity measures imposed on it by the EU would go ahead.
"One thing remains certain," he said. "The refusal of a [debt payment] extension of a few short days, and the attempt to cancel a purely democratic process is an insult and a great disgrace to Europe’s democratic traditions."
He finished: "The dignity of the Greek people in the face of blackmail and injustice will send a message of hope and pride to all of Europe."
There must be a "viable" solution to Greece's debt crisis, the country's prime minister has said as he arrived for crunch talks with European leaders.
"This is time for a substantial, viable solution that allows Greece to come back to growth within the eurozone with social justice," Alexis Tsipras said as he arrived in Brussels.
Speaking alongside Mr Tsipras, European Commission president Jean-Claude Juncker said there had been progress but added: "We are not there yet".
Greece has to reach a deal with its creditors before a June 30 deadline to repay a 1.6 billion euro (£1.1 billion) loan from the International Monetary Fund (IMF).
Mr Tsipras's left-wing Syriza government has presented new proposals for tackling the crisis which have to be approved before it can gain access to vital bailout funds.
Greek Socialist leader Evangelos Venizelos has rejected joining a coalition led by a leftist party opposed to an international bailout.
However, said he would continue efforts to form a government when he receives a mandate from the country's president.
After meeting with Left Coalition leader Alexis Tsipras, Venizelos said:
Greece's leader of the Left Coalition party Alexis Tsipras has ruled out entering a coalition with the conservative New Democracy party, and said he would try to form a government with other leftist parties.
Speaking after talks with New Democracy leader Antonis Samaris, Tsipras said:
Greece's general election has turned the spotlight on a new political figure.
The 37-year-old leader of the Left Coalition party, Alexis Tsipras, had 15.8% with almost 40% of the vote counted, beating the socialist PASOK into second place so far.
Tsipras ran on an anti-austerity ticket arguing that the bailout or bust ultimatum thrown down by mainstream parties was a scare tactic to force people to accept cuts.