Supporters of opposition leader and head of radical leftist Syriza party Alexis Tsipras cheer at exit poll results in Athens, January 25, 2015. [Photo/Agencies]

Tsipras has promised to renegotiate agreements with the European Commission, European Central Bank and International Monetary Fund "troika" and write off much of Greece's 320 billion-euro debt, which at more than 175 percent of gross domestic product, is the world's second highest after Japan.

Coming after the ECB's move to pump billions into the bloc's flagging economy, Sunday's result will stir consternation in Berlin. A senior lawmaker in Merkel's conservative party said the result showed Greek voters had turned away from austerity but he said Europe could not accept rejection of the bailout.

"We must not reward the breaching of agreements," Wolfgang Bosbach told the daily Osnabruecker Zeitung newspaper. "That would send completely the wrong signal to other crisis-stricken countries that would then expect the same treatment."

Tsipras wants to roll back many of the measures demanded by the "troika", raising the minimum wage, lowering power prices for poor families, cutting property taxes and reversing pension and public sector pay cuts.

US investment bank J.P. Morgan said the result could weigh on markets but that it considered speculation over a possible Greek exit from the euro was "a stretch" and a negotiated deal appeared the most likely outcome. "Our base case remains that a Syriza government or Syriza-dominated coalition would alter its platform to retain troika financing," it said.

Greece, unable to tap the markets because of sky-high borrowing costs, has enough cash to meet its immediate funding needs for the next couple of months but it faces around 10 billion euros of debt repayments over the summer.