ATHENS — Greece rejected the harsh economics of austerity on Sunday and sent a warning to the rest of Europe as the left-wing Syriza party won a decisive victory in national elections, positioning its tough-talking leader, Alexis Tsipras, to become the next prime minister.

With almost 98 percent of the vote counted, Syriza had 36 percent, almost nine points more than the governing center-right New Democracy party of Prime Minister Antonis Samaras, who conceded defeat. The only uncertainty was whether Syriza would muster a parliamentary majority on its own or have to form a coalition.

Appearing before a throng of supporters outside Athens University late Sunday, Mr. Tsipras, 40, declared that the era of austerity was over and promised to revive the economy. He also said his government would not allow Greece’s creditors to strangle the country.

“Greece will now move ahead with hope and reach out to Europe, and Europe is going to change,” he said. “The verdict is clear: We will bring an end to the vicious circle of austerity.”