MOSCOW — Vladimir Putin swept Sunday’s presidential election to return to the Kremlin and extend his hold over Russia for six more years, incomplete returns showed. His eyes brimming with tears, he defiantly proclaimed to a sea of supporters that they had triumphed over opponents intent on “destroying Russia’s statehood and usurping power.”

Putin’s win was never in doubt as many across the vast country still see him as a guarantor of stability and the defender of a strong Russia against a hostile world, an image he has carefully cultivated during 12 years in power.

Accounts by independent observers of extensive vote-rigging, however, looked set to strengthen the resolve of opposition forces whose unprecedented protests in recent months have posed the first serious challenge to Putin’s heavy-handed rule. Another huge demonstration was set for Monday evening in central Moscow.

With fewer than a quarter of the votes counted, Putin spoke to tens of thousands of supporters at a rally just outside the Kremlin walls. Many of them were government workers or employees of state-owned companies who had been ordered to attend.

Putin, 59, said the election showed that “our people can easily distinguish a desire for renewal and revival from political provocations aimed at destroying Russia’s statehood and usurping power.”

The wave of protests began after a December parliamentary election in which observers produced evidence of widespread vote fraud. Protest rallies in Moscow drew tens of thousands in the largest outburst of public anger in post-Soviet Russia, demonstrating growing exasperation with the pervasive corruption and tight controls over political life under Putin.

Golos, Russia’s leading elections watchdog, said it received numerous reports of “carousel voting,” in which busloads of voters are driven around to cast ballots multiple times.