A ballot box is emptied by a voting official at the closing of polling stations in Athens, Greece July 5, 2015. [Photo/Agencies]

But European officials and most of Greece's opposition parties painted the referendum as one of whether the country kept using the euro currency - even though that was not the convoluted question asked on the ballot. Opinion polls Friday showed that 74 percent or more want their country to remain in the euro.

"Given the unfavorable conditions last week, you have made a very brave choice," Tsipras told Greeks in his address. "But I am aware that the mandate you gave me is not a mandate for rupture." He said he would seek to negotiate a viable solution with the country's creditors.

How European officials react to the referendum result will be critical for the country, and a eurozone summit was called for Tuesday evening to discuss the situation.

German Chancellor Angela Merkel and French President Francois Hollande spoke to each other Sunday night and agreed "that the vote of the Greek people must be respected," Merkel's office said.

The referendum result was "very regrettable for the future of Greece," said Jeroen Dijsselbloem, head of the eurozone finance ministers' meeting known as the Eurogroup, which also will meet Tuesday.

Dijsselbloem, who is finance minister for the Netherlands, had been a steadfast opponent of Greece as it sought better conditions during five months of bailout talks.

"For recovery of the Greek economy, difficult measures and reforms are inevitable," he said. "We will now wait for the initiatives of the Greek authorities."

Sigmar Gabriel, Germany's vice chancellor and economic minister, told a German newspaper the Greek government was leading its people "onto a path of bitter austerity and hopelessness."

Tsipras has "torn down the last bridges, across which Europe and Greece could move toward a compromise," Gabriel told the daily Tagesspiegel. "By saying 'no' to the eurozone's rules, as is reflected in the majority 'no' vote, it's difficult to imagine negotiations over an aid package for billions."

Belgian Finance Minister Johan Van Overtveldt was somewhat softer in his reaction, saying a "no" result "complicates matters," but that the door was open to resume talks immediately.

"What we certainly don't want to do is to take decisions that will threaten the monetary union," he told Belgium's VRT. "Within that framework we can start talks again with the Greek government, literally, within hours."

Time has run out for Greece, which is dealing with an economy in a protracted recession, with high unemployment and banks dangerously low on capital.

The international bailout - under which it received nearly 240 billion euros in rescue loans - expired last week, on the same day Greece defaulted on an IMF repayment, becoming the first developed nation to do so.

Of critical importance will be whether the European Central Bank decides to maintain its lifeline to Greece in the form of emergency liquidity assistance, or ELA. The assistance, now at around 90 billion euros, has been maintained but not increased in past days, leaving the country's financial system in a stranglehold.