KHARKIV, Ukraine — Ukrainian troops retook a wide swath of territory from Russia on Monday, pushing all the way back to the northeastern border in some places, and claimed to have captured many Russian soldiers as part of a lightning advance that forced Moscow to make a hasty retreat.

A spokesman for Ukrainian military intelligence said Russian troops were surrendering en masse as “they understand the hopelessness of their situation.” A Ukrainian presidential adviser said there were so many POWs that the country was running out of space to accommodate them.

As blue-and-yellow Ukrainian flags fluttered over newly liberated towns, the Ukrainian military said it had freed more than 20 settlements in 24 hours. In recent days, Kyiv’s forces have captured territory at least twice the size of greater London, according to the British Defense Ministry.

Ukraine Morale Soars

After months of little discernible movement on the battlefield, the momentum has lifted Ukrainian morale and provoked rare public criticism of Russian President Vladimir Putin’s war.

“In some areas of the front, our defenders reached the state border with the Russian Federation,” said Oleh Syniehubov, governor of the northeastern Kharkiv region. Over the weekend, the Russian Defense Ministry said troops would be pulled from two areas in that region to regroup in the eastern region of Donetsk.

There were reports of chaos as Russian troops pulled out.

“The Russians were here in the morning. Then at noon, they suddenly started shouting wildly and began to run away, charging off in tanks and armored vehicles,” Dmytro Hrushchenko, a resident of recently liberated Zaliznychne, a small town near the eastern front line, told Sky News.

Video taken by the Ukrainian military showed soldiers raising the Ukrainian flag over battle-damaged buildings. In one scene, a fighter wiped his boots on a Russian flag on the ground. Other videos showed Ukrainians inspecting the wreckage of Russian military vehicles, including tanks.

Ukrainians Disarming Land Mines

Ukrainian teams were disarming land mines and other unexploded weapons in the recaptured areas and searching for any remaining Russian troops, officials said.

It was not yet clear if the Ukrainian blitz could signal a turning point in the war. Momentum has switched back and forth before, but rarely with such a big and sudden swing.

Ukrainian presidential adviser Oleksiy Arestovich did not specify the number of Russian prisoners but said the POWs would be exchanged for Ukrainian service members held by Moscow. Military intelligence spokesman Andrey Yusov said the captured troops included “significant” numbers of Russian officers.

Ukraine’s deputy interior minister accused fleeing Russian forces of burning official documents and concealing bodies in an attempt to cover up rights violations in the areas they controlled until last week.

The mood was jubilant across the country.

Power and Water Restored in Kharkiv

In Kharkiv, Ukraine’s second-largest city and the capital of the region where the gains have been made, authorities hailed that power and water had been restored to about 80% of the region’s population following Russian attacks on infrastructure that knocked out electricity in many places across Ukraine.

“You are heroes!!!” Kharkiv Mayor Ihor Terekhov wrote on Telegram, referring to crews who restored utilities in Ukraine’s second-biggest city. “Thanks to everyone who did everything possible on this most difficult night for Kharkiv to normalize the life of the city as soon as possible.”

The buoyant mood was also captured by a defiant President Volodymyr Zelenskyy late Sunday on social media.

“Do you still think you can intimidate, break us, force us to make concessions?” Zelenskyy asked. “Cold, hunger, darkness, and thirst for us are not as scary and deadly as your ‘friendship’ and brotherhood.’”

In the end, he exclaimed: “We will be with gas, lights, water and food … and WITHOUT you!”

Russians Criticize Putin

Meanwhile, in Russia, there were some signs of disarray as Russian military bloggers and patriotic commentators chastised the Kremlin for failing to mobilize more forces and take stronger action against Ukraine.

Russia has continuously stopped short of calling its invasion a war, instead describing it as a “special military operation” and relying on a limited contingent of volunteers instead of a mass mobilization that could spur civil discontent and protest.

Ramzan Kadyrov, the Moscow-backed leader of the Russian region of Chechnya, publicly criticized the Russian Defense Ministry for what he called “mistakes” that made the Ukrainian blitz possible.

Even more notable, such criticism seeped onto state-controlled Russian TV.

“People who convinced President Putin that the operation will be fast and effective … these people really set up all of us,” Boris Nadezhdin, a former parliament member, said on a talk show on NTV television. “We’re now at the point where we have to understand that it’s absolutely impossible to defeat Ukraine using these resources and colonial war methods.”

The Washington-based Institute for the Study of War said Monday that Kyiv “will likely increasingly dictate the location and nature of the major fighting.”

The British Defense Ministry said the retreat would likely further deteriorate the trust Russian forces have in their commanders and put Moscow’s troops on the back foot.