Not long after Russian tanks rolled into Ukraine, its soldiers broke down the office door of Melitopol’s mayor, Ivan Fedorov. They put a bag over his head, bundled him into a car, and drove him around the southern city for hours, threatening to kill him.

Mr. Fedorov, 34, is one of over 50 local leaders who have been held in Russian captivity since the war began on February 24 in attempts to subdue cities and towns coming under Moscow’s control. Like many others, he said he was pressured to collaborate with the invaders.

“The bullying and threats did not stop for a minute. They tried to force me to continue leading the city under the Russian flag, but I refused,” Mr. Fedorov told the Associated Press by phone last month in Kyiv. “They didn’t beat me, but day and night, wild screams from the next cell would tell me what was waiting for me.”

As Russians seized parts of eastern and southern Ukraine, civilian administrators and others, including nuclear power plant workers, say they have been abducted, threatened, or beaten to force their cooperation — something that legal and human rights experts say may constitute a war crime.

Ukrainian and Western historians say the tactic is used when invading forces are unable to subjugate the population. This year, as Russian forces sought to tighten their hold on Melitopol, hundreds of residents took to the streets to demand Mr. Fedorov’s release. After six days in detention and an intervention from President Zelensky, he was exchanged for nine Russian prisoners of war and expelled from the occupied city. A pro-Kremlin figure was installed.

“The Russians cannot govern the captured cities. They have neither the personnel nor the experience,” Mr. Fedorov said. They want to force public officials to work for them because they realize that someone has to “clean the streets and fix up the destroyed houses.”

The Association of Ukrainian Cities, a group of local leaders from across Ukraine, said that of the more than 50 abducted officials, including 34 mayors, at least 10 remain captive. Russian officials haven’t commented on the allegations. Moscow-backed authorities in eastern Ukraine even launched a criminal investigation into Mr. Fedorov on charges of involvement in terrorist activities.

“Kidnapping the heads of villages, towns, and cities, especially in wartime, endangers all residents of a community, because all critical management, provision of basic amenities, and important decisions on which the fate of thousands of residents depends are entrusted to the community’s head,” Kyiv’s mayor, Vitali Klitschko, head of the AUC, said.

In the southern city of Kherson, one of the first seized by Russia and a key target of an unfolding counteroffensive, Mayor Ihor Kolykhaiev tried to stand his ground. He said in April that he would refuse to cooperate with its new, Kremlin-backed overseer.

The deputy head of the Russian-installed regional administration, Kirill Stremousov, repeatedly denounced Mr. Kolykhaiev as a “Nazi,” echoing the false Kremlin narrative that its attack on Ukraine was an attempt to “de-Nazify” the country.

Mr. Kolykhaiev continued to supervise Kherson’s public utilities until his arrest on June 28. His whereabouts remain unknown.

According to the UN Human Rights Monitoring Mission in Ukraine, 407 forced disappearances and arbitrary arrests of civilians were recorded in areas seized by Russia in the first six months of the war. Most were civil servants, local councilors, civil society activists, and journalists.

A senior researcher at Human Rights Watch, Yulia Gorbunova, said the abuse “violates international law and may constitute a war crime,” adding that Russian forces’ actions appeared to be aimed at “obtaining information and instilling fear.”

The UN human rights office has warned repeatedly that arbitrary detentions and forced disappearances are among possible war crimes committed in Ukraine.

Several mayors have been killed, shocking Ukrainian society. Following the discovery of mass burials in areas recaptured by Kyiv, Ukrainian and foreign investigators continue to uncover details of extrajudicial killings of mayors.

Ukraine’s government has tried to swap captive officials for Russian POWs, but officials complain that Moscow sometimes demands Kyiv release hundreds for each Ukrainian in a position of authority, prolonging negotiations. Each week brings reports of abductions of officials, engineers, doctors, and teachers who won’t cooperate with the Russians.

A history professor at Cambridge University,  Hubertus Jahn, said that from the time of Peter the Great onward, the tactic by imperialist Russia of co-opting locals targeted elites and nobility, with resistance often bringing Siberian exile. Mr. Jahn called it an obvious strategy “if you don’t have the strength to subordinate a region outright.”