Alexis Tsipras, president of the Synaspismos political party and head of the Coalition of the Radical Left parliamentary group, known as SYRIZA -- which became Greece's second-biggest party in May 6 elections -- said it made no sense to participate in further meetings because weeklong coalition talks with the same party leaders were deadlocked.
SYRIZA wants Greece to tear up a $168 billion loan deal with its European partners and its associated austerity measures. The deal was struck in March, and is designed to keep the country afloat and inside the eurozone.
Eurozone finance ministers have warned failing to abide by the deal's terms would mean Greece risks losing any future financial aid and could be drummed out of the eurozone.
"The denial to take part in a coalition government is not from SYRIZA but from the Greek people," Tsipras was quoted in The Wall Street Journal as saying last week. "The [loan agreement] has already been denounced by the Greek people."
Tsipras, 37, has said he wants to keep Greece in the eurozone but calls the bailout's terms criminal.
"They are not asking for agreement -- they are asking us to be their partners in crime and we will not be their accomplices," Tsipras said Sunday after meeting with conservative and socialist leaders, who support the deal's austere terms.
President Karolos Papoulias must call for new general elections next month if a coalition can't be formed.
If Papoulias calls for new elections, SYRIZA would likely win them, analysts say.
A poll published in To Vima newspaper Sunday showed SYRIZA was now Greece's most popular party, soaring after the May 6 elections, while the conservative New Democracy and socialist PASOK parties ranked second and third, respectively.
SYRIZA and five other parties that campaigned against the austerity program already won more than 60 percent of the May 6 vote.
Eurozone finance ministers were to meet in Brussels Monday to discuss Greece, as well as a quickly worsening crisis in Spain and a political about-face in France to economic-growth policies from belt-tightening.
All 27 European Union finance ministers are to meet Tuesday -- the same day incoming French President Francois Hollande -- who defeated Nicolas Sarkozy by championing economic growth -- is to meet with German Chancellor Angela Merkel, who favors austerity.
Merkel's conservative Christian Democratic Union lost in an election Sunday in North Rhine-Westphalia, Germany's most populous state, with the opposition Social Democrats and Greens securing an absolute majority in results indicated early Monday.
The CDU slumped to 26 percent, its worst result in the state since World War II.
Political analysts cautioned against seeing the results as a general-election bellwether, noting Merkel and the CDU still led popularity polls in most of Germany, the Financial Times reported.
Despite Sunday's loss and Hollande's political momentum, Merkel is unlikely to abandon her tough line on austerity, arguing Europe's debt crisis cannot be fixed by piling up more debt, the British newspaper The Guardian reported.
The cover of Monday's German news weekly Der Spiegel says, "Adieu Greece," arguing it is time to kick the country out of the euro.
Members of the European Central Bank's governing council said over the weekend a Greek eurozone departure would not necessarily be fatal to the eurozone -- a reversal from ECB President Mario Draghi comments to the Financial Times Dec. 18 that a Greek eurozone exit would have incalculable consequences.
Weekend protests brought more than 72,000 protesters to five major Spanish cities, including Madrid and Barcelona.
The protests were peaceful, despite concerns they would turn violent, El Pais reported.
Spain will miss its budget-deficit targets for this year and next, the government said Friday.
Its deficit will be 6.4 percent of gross domestic product this year and 6.3 percent next year, the government said.
It had promised the European Commission, the EU's executive body, to cut its deficit to 5.3 percent of GDP this year and to 3 percent next year, a limit to be required of all EU countries then.