The architects of Greece's international bailout struggled to respond to a resounding "no" in Sunday's referendum on the austerity measures they had tied to the aid program.

Eurozone leaders said they would meet in Brussels Tuesday evening to discuss the next steps. But initial reactions suggested that the 18 other eurozone members may have difficulties sticking to the common line they have drawn over the past five months: that without more budget cuts and pension overhauls, Greece shouldn't expect any more bailout loans.

"This result is very regrettable for the future of Greece," said Jeroen Dijsselbloem, the Dutch finance minister who presides over the talks with his eurozone counterparts. "For recovery of the Greek economy, difficult measures and reforms are inevitable. We will now wait for the initiatives of the Greek authorities."

Some of Mr. Dijsselbloem's colleagues were even harsher toward the Greeks.

The "rejection of reforms cannot mean that they will get the money easier," Slovakia's finance minister, Peter Kazimir, said in a Twitter message. That, he warned in a separate tweet, could soon push Greece out of the currency union: "The nightmare of the 'euro architects' that a country could leave the club seems like a realistic scenario after Greece voted No today."

But other eurozone policy makers were still looking for ways to keep Greece in their midst. "Italy has always worked for a supportive and more integrated Europe. It was true yesterday, and it will still be true tomorrow," Italian Economy Minister Pier Carlo Padoan said in a Twitter message.

Earlier Sunday, before polling stations had closed, France's economy minister also signaled openness for more talks. "Even if the Greeks vote no, our responsibility tomorrow will be to not do a Versailles Treaty for the eurozone," Emmanuel Macron said, referring to the treaty at the end of World War I that imposed heavy sanctions on Germany.

"It's not because one side thinks they've won a referendum that we can crush a people," Mr. Macron added.

The European Commission, the European Union's executive arm that has helped police rescue loans for Greece over the past five years, said Commission President Jean-Claude Juncker "is consulting tonight and tomorrow with the democratically elected leaders of the other 18 eurozone members as well as with the heads of the EU institutions."

An EU official said that Mr. Juncker hadn't reached out to Greek Prime Minister Alexis Tsipras and that no talks on further bailout aid were taking place. "They will see each other at the euro summit," the official said. But German Chancellor Angela Merkel and French President Franç ois Hollande both spoke to Mr. Tsipras Sunday night.

Antiausterity and anti-euro parties from the right and the left meanwhile hailed the vote as a victory for their own ideas.

"Today democracy has won in Greece," Pablo Iglesias, the leader of Spain's leftist Podemos party said in a tweet. Pablo Echenique, head of the party in the northern region of Aragon, wrote that "our Greek brothers just told the financial dictators that democracy and dignity are not to be tampered with."

France's populist parties also celebrated the "no" vote. The far-right National Front, which combines anti-immigration and anti-euro policies, said the Greek referendum is a victory for democracy and the Greek people. "I think we're seeing the beginning of the end of the eurozone," Florian Philippot, vice president of the National Front, said on French television channel France 2.

France's far-left political groups, which have closer ties to ruling party Syriza, also cheered the result. Jean-Luc Mé lenchon, the leftist politician who took more than 11% of the vote in 2012 elections, said the time had come to write off Greece's debt and abandon austerity.

"The Greeks have held their heads high so from now on we can start talking seriously in Europe. The absurd policies of Mme. Merkel and [German Finance Minister Wolfgang] Schä uble have just been beaten," Mr. Mé lenchon said on BFMTV.

Andrea Thomas in Berlin, Giovanni Legorano in Milan, Giada Zampano in Rome and David Romá n in Madrid contributed to this article.

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