But despite fears of a "Grexit," Greek Prime Minister Alexis Tsipras said the result was not a mandate to clash with Europe.
Speaking after the majority of votes had been counted, Tsipras said Greeks hadn't voted on whether or not to stay in Europe, and that the government would return to the negotiating table with creditors on Monday.
Stelios Kouloglou, an MEP with Greece's ruling Syriza party, told CNBC that nobody had predicted this result.
"Even with the banks closed, people can't get money anywhere - and still, more than 60 percent voted to support the government. This is a huge vote," he said on Sunday.
Following news of the "no" victory, opposition leader Antonis Samaras stepped down.
Kouloglou said the deal proposed by creditors—and voted on by Greeks in this referendum—was "not practical."
With the result, Tsipras "has more strength to negotiate. The real (aim) for us is not to leave the euro zone, but to negotiate a better deal," he added.
Before polls closed, Greek Finance Minister Yanis Varoufakis told CNBC that the government could potentially reach a deal with lenders within 24 hours if the "no" campaign was successful.