Ukraine’s troops entered the southern port city of Kherson on Friday, its military said, putting Kyiv on the cusp of achieving one of the most significant victories of the war. It was a bitter blow to President Vladimir V. Putin, who just a month ago declared Kherson a part of Russia forever. Videos shared by Ukrainian government officials on social media showed scenes of jubilant residents waving Ukrainian flags outside government buildings, cheering the arrival of a contingent of Ukrainian troops shortly after Russian forces retreated across the Dnipro river.

“Kherson is returning under the control of Ukraine, units of the Armed Forces of Ukraine are entering the city,” the Ukrainian military intelligence agency said in a statement.

Hours earlier, Russia’s defence ministry had said in a statement that the retreat of all troops to the eastern bank of the Dnipro river was complete.

The loss of Kherson is Russia’s third major setback of the war, following retreats from Kyiv, the capital, last spring, and from the Kharkiv region in the northeast in September. Kherson was the only provincial capital Russia had captured since invading in February, and it was a major link in Russia’s effort to control the southern coastline along the Black Sea.

The success in Kherson will also bolster the Ukrainian government’s argument that it should press on militarily while it has Russian forces on the run, and not return to the bargaining table, as some American officials have advocated.

Photographs shared with The New York Times by residents showed several dozen people outside the main government building waving flags and wrapping the base of a statue outside the building in blue and gold. They added a flag of the EU above the pedestal where a statue of Lenin once stood.

The dramatic scenes in Kherson came less than 48 hours after Russia’s defence minister announced that Russian troops in the city would withdraw. After ordering all civilians to leave towns and villages in the region last month and engaging in a systematic effort to loot museums, government buildings, hospitals and homes, Russian forces this week blew up bridges and laid a vast network of mines to slow the Ukrainian advance as they abandoned long-held defensive positions west of Kherson city.

Even as its soldiers fled, the Kremlin said that it still considered Kherson — which President Vladimir V. Putin illegally annexed in September — to be a part of Russia.

“This is a Russian region,” Dmitri S. Peskov, the Kremlin’s spokesman, told reporters on Friday. “It has been legally fixed and defined. There can be no changes here.”

As he spoke, Ukrainian soldiers continued to move through towns and villages in the region, greeted joyously by tearful residents who had endured nine months of occupation.

Oleh Voitsehovsky, the commander of a Ukrainian drone reconnaissance unit, said he had seen no Russian troops or equipment in his zone along the front less than four miles north of Kherson city. “The Russians left all the villages,” he said.

“We looked at dozens of villages with our drones and didn’t see a single car. We don’t see how they are leaving. They retreat quietly, at night.”

But the remaining residents of Kherson were still awaiting the arrival of Ukrainian troops. The apparent final hours of the Russian occupation overnight on Thursday to Friday featured several explosions and were chaotic and disorienting, according to residents of Kherson.

(New York Times News Service)