ATHENS — Greeks delivered a shocking rebuff to Europe’s leaders on Sunday, decisively rejecting a deal offered by the country’s creditors in a historic vote that could redefine Greece’s place in Europe and shake the Continent’s financial stability.

As people gathered to celebrate in Syntagma Square in central Athens, the Interior Ministry reported that with more than 90 percent of the vote tallied, 61 percent of the voters had said no to a deal that would have imposed greater austerity measures.

The no votes carried virtually every district in the country, handing a sweeping victory to Prime Minister Alexis Tsipras, a leftist who came to power in January vowing to reject new austerity measures, which he called an injustice and economically self-defeating. Last month he walked away from negotiations in frustration at the creditors’ demands, called the referendum and urged Greeks to vote no as a way to give him more bargaining power.

While Mr. Tsipras now appears to have gotten his wish, his victory in the referendum settled little, since the creditors’ offer is no longer on the table. There remains the possibility that they could walk away, leaving Greece facing default, financial collapse, and expulsion from the eurozone and, in the worst case, from the European Union.