Russian forces have looted tens of thousands of pieces, including avant-garde oil paintings and Scythian gold. Experts say it is the biggest art heist since the Nazis in World War II, intended to strip Ukraine of its cultural heritage.

One morning in late October, Russian forces blocked off a street in downtown Kherson and surrounded a graceful old building with dozens of soldiers.

Five large trucks pulled up. So did a line of military vehicles, ferrying Russian agents who filed in through several doors. It was a carefully planned, highly organized, military-style assault — on an art museum.

Over the next four days, the Kherson Regional Art Museum was cleaned out, witnesses said, with Russian forces “bustling about like insects,” porters wheeling out thousands of paintings, soldiers hastily wrapping them in sheets, art experts barking out orders and packing material flying everywhere.

“They were loading such masterpieces, which there are no more in the world, as if they were garbage,” said the museum’s longtime director, Alina Dotsenko, who recently returned from exile, recounting what employees and witnesses had told her.

When she came back to the museum in early November and grasped how much had been stolen, she said, “I almost lost my mind.”

Kherson. Mariupol. Melitopol. Kakhovsky. Museums of art, history and antiquities.

As Russia has ravaged Ukraine with deadly missile strikes and brutal atrocities on civilians, it has also looted the nation’s cultural institutions of some of the most important and intensely protected contributions of Ukraine and its forebears going back thousands of years.

International art experts say the plundering may be the single biggest collective art heist since the Nazis pillaged Europe in World War II.

In Kherson, in Ukraine’s south, Ukrainian prosecutors and museum administrators say the Russians stole more than 15,000 pieces of fine art and one-of-a-kind artifacts. They dragged bronze statues from parks, lifted books from a riverside scientific library, boxed up the crumbling, 200-year-old bones of Grigory Potemkin, Catherine the Great’s lover, and even stole a raccoon from the zoo, leaving behind a trail of vacant cages, empty pedestals and smashed glass.

Ukrainian officials say that Russian forces have robbed or damaged more than 30 museums — including several in Kherson, which was retaken in November, and others in Mariupol and Melitopol, which remain under Russian occupation. With Ukrainian investigators still cataloging the losses of missing oil paintings, ancient steles, bronze pots, coins, necklaces and busts, the number of reported stolen items is likely to grow.

The plundering is hardly a case of random or opportunistic misbehavior by a few ill-behaved troops, Ukrainian officials and international experts say, or even a desire to turn a quick profit on the black market. Instead, they believe the thefts are a broadside attack on Ukrainian pride, culture and identity, consistent with the imperial attitude of Russia’s president, Vladimir V. Putin, who has constantly belittled the idea of Ukraine as a separate nation and used that as a central rationale for his invasion.

“It’s not like one soldier putting a silver chalice in his rucksack,” said James Ratcliffe, general counsel of The Art Loss Register, a London-based organization that traces stolen art. “This is a far, far larger scale.”

At one museum in Melitopol, a southern Ukrainian city that the Russians seized in the first days of the war, witnesses said that a mysterious man in a white lab coat had arrived to carefully extract, with gloves and tweezers, the most valuable objects from the collection, including gold pieces from the Scythian empire crafted 2,300 years ago. As he lifted out the priceless antiquities, a squad of Russian soldiers stood firmly behind him, in case anyone should try to stop him.

In each case of looting, witnesses — including caretakers, security guards and other museum employees, who said they had been pressured or forced to help — reported a centrally controlled expert-led operation.

“Shocked is not the word. I am furious,” Oleksandr Tkachenko, Ukraine’s culture minister, said in a broadcast interview as he toured the looted Kherson art museum, visibly upset. “If they stole our heritage, they believe that we wouldn’t continue to live and to create. But we will.”

The Ukrainians have a lot of battles on their hands. Towns in the east like Bakhmut are being pummeled. Drone swarms continue to take out critical infrastructure, plunging thousands into the dark. Vast swaths of territory in the south and east remain occupied, and one out of three Ukrainians has been forced to flee from home.

But even with the war raging, a group of Ukrainian lawyers and art experts are working day and night to collect evidence for what they hope will be future prosecutions of cultural crimes. From dimly lit offices in frosty buildings with no power or heat, wearing gloves and woolly hats indoors, they make meticulous lists of missing objects, comb through museum records and try to identify potential witnesses and local collaborators who might have helped the Russians steal.

The Ukrainians are also working with international art organizations, like The Art Loss Register, to track the looted pieces.

“Everyone in the art market is on red alert to look out for this material,” Mr. Ratcliffe said. “Every auction house that sees material from Ukraine is going to start asking a lot of questions.”

His organization, he said, has already registered more than 2,000 items from Ukraine believed to have been stolen and others at risk, including paintings from Kherson’s art museum and Scythian gold from Melitopol.

The Ukrainians accuse the Russians of breaking international treaties that outlaw art looting, such as the 1954 Convention for the Protection of Cultural Property in the Event of Armed Conflict. Created in the wake of World War II, the treaty calls for signatories to “prohibit, prevent and, if necessary, put a stop to any form of theft” of cultural property. Both Ukraine and Russia signed it.

But the Russians have flipped the narrative and presented their actions not as theft but liberation.

“Don’t panic,’’ said Kirill Stremousov, Kherson’s Russia-installed deputy administrator, when he explained in October what had happened to the statues that disappeared from Kherson. He said that when the fighting stopped, the monuments would “definitely return,” and that “everything was being done for the benefit of preserving the historical heritage of the city of Kherson.”

The statues have yet to be returned. (And a few weeks later, just as Ukrainian troops were liberating Kherson, Mr. Stremousov was killed in a suspicious car crash.)

Many of the paintings looted from the Kherson art museum, including beloved classics like “Piquet on the Bank of the River. Sunset,” by the miniaturist Ivan Pokhytonov, and “Autumn Time,” by Heorhii Kurnakov, recently showed up at a museum in Crimea, the Black Sea peninsula that Russia snatched from Ukraine in 2014.

The director of the museum, Andrei Malguin, offered a familiar rationale. “We have 10,000 pieces and we are inventorying them,” he told a Spanish newspaper, El País. He said his museum was keeping the collection for its own “protection.”

(Russian soldiers similarly displayed the four-legged booty they had “liberated” from Kherson’s zoo. In videos that went viral everywhere, paratroopers declared that the stolen raccoon was now their mascot, traveling with them along the front, and had been named Kherson. That led to a popular meme on the Ukrainian internet: Saving Private Raccoon).

This is hardly the first time that Russia has interfered with Ukrainian art or culture. For hundreds of years during imperial Russia and then in the 20th century during Soviet times, Moscow constantly tried to suppress the Ukrainian language and anything that would bolster Ukrainian identity.

After Russia grabbed Crimea, Interpol, the international police organization, said that it was searching for 52 paintings by Ukrainian artists that had been illegally transferred to an art museum in Simferopol, Crimea’s second-largest city, in March 2014.

So this time, when war erupted in February, Ukrainian officials were quick to wrap outdoor statues in sheaths of sandbags and move precious works of art into underground vaults. But the Russians were not so easily deterred.

In Melitopol, Russian soldiers kidnapped the art museum’s director and a caretaker and eventually found the Scythian gold hidden in cardboard boxes in the cellar.

In Kherson, after Ms. Dotsenko fled for Kyiv, pro-Russia collaborators took over the art museum. Ukrainian officials said that in August, a well-dressed delegation from Crimean museums had arrived to scout out the goods.

But they didn’t have much time. Ukrainian forces pressed in from three sides. By October, Russia’s hold on Kherson was unraveling faster than anyone expected. At the art museum, Russian agents rushed to get everything out as fast as possible.

“The removal took place with the participation of museum specialists but with gross violations of the transportation and packaging of the works,” said Vitalii Tytych, a Ukrainian lawyer who is part of a special military unit documenting war crimes against the cultural heritage of Ukraine. “Paintings were taken out of the frames in a hurry, frames were broken, cultural objects were also damaged or destroyed.”

“Many works,” he lamented, “will be lost.”

Touring Kherson’s museums now is depressing. Virtually all of the thousands of oil paintings that had been stowed in the art museum’s basement — and the computer records documenting them ­— are gone.

“I am the daughter of an officer who raised me to be strong, but I cried for two weeks,” said Ms. Dotsenko, who has worked at the art museum for 45 years.

“No,” she corrected herself, “I didn’t cry, I sobbed. I bit the walls. I gnawed.”

Across the street, at the Kherson Museum of Local Lore, there is one shattered display case after another. Deep gouges have been cut into the floor from soldiers dragging out centuries-old artifacts. Sometimes they didn’t succeed. Denys Sykoza, an inspector of cultural objects for the Kherson government, stood in front of the remains of a delicate glass cup from the fifth century, staring at the shards.

“They broke this trying to steal it,” he said quietly. “And there was only one like it.”