The authorities in Poland and Germany have arrested at least five of their citizens in recent days and accused them of spying for Russia or of offering to help Moscow commit violence on European soil, including a “possible attack” on the president of Ukraine, Volodymyr Zelensky.

The arrests underscored fears of the Kremlin’s furtive network in the West and its use of foreign nationals, including violent criminals and soccer hooligans, to terrify or possibly even kill opponents sheltering abroad.

The Polish National Prosecutor’s Office said in a statement that a Polish citizen, identified as Pawel K., was detained on Wednesday. It said he had offered to assist Russian agents in a possible plot aimed at killing Mr. Zelensky.

It gave few details, other than saying he had “declared his readiness to act for the military intelligence of the Russian Federation and established contacts with citizens of the Russian Federation directly involved in the war in Ukraine.”

Prime Minister Donald Tusk of Poland said on Friday that the police had also arrested two Polish citizens for attacking a top aide to the Russian opposition campaigner Aleksei A. Navalny outside the aide’s home in Vilnius, the capital of Lithuania, last month with a hammer. The Polish leader said a citizen of Belarus, who allegedly had ordered up the attack on Moscow’s behalf, had also been apprehended. Mr. Navalny died in Russian prison in February.

“There will be no leniency for collaborators with the Russian services,” Mr. Tusk said in his statement on Friday. He vowed to stamp out “every betrayal and attempt at destabilization.”

The Lithuanian prosecutor general’s office said in a statement that the two Poles suspected of assaulting Mr. Navalny’s aide, Leonid Volkov, on March 12 were detained in Warsaw on April 3. The statement did not mention a Belarusian.

Poland’s national prosecutor’s office said that the man, named Pawel K., had been given tasks that “included collecting and providing information” about Rzeszów-Jasionka Airport, a Polish airfield near the border with Ukraine through which Mr. Zelensky often travels on his trips abroad.

Dmitri S. Peskov, the Kremlin spokesman, declined to comment Friday during a call with reporters on the possible plot to assassinate Mr. Zelensky.

The airport, which is protected by Patriot missiles operated by U.S. military personnel, serves as an important transport hub for Western weapons destined for Ukraine.

The Ukrainian intelligence service said in a statement that it had given information to the Polish authorities about the individual who was arrested. It added that his plan had been to collect and transfer information to Russian military intelligence about the airport in order to “help the Russian special services plan a possible assassination attempt on the President of Ukraine Volodymyr Zelensky during his stay in Poland.”

The arrests in Poland came as the German authorities apprehended two dual citizens of Germany and Russia on suspicion of spying for Moscow and plotting to sabotage aid to Ukraine by blowing up Western military infrastructure.

The string of arrests comes as many European officials worry about the extent to which Moscow has infiltrated countries in the European Union with spies and assassins, and has found an accessible network of men for hire among organized crime and extremist groups, willing to carry out assaults and acts of intimidation against the Kremlin’s foes.

Mr. Tusk said the two Polish men who had attacked Mr. Volkov with a hammer ran in “ultra” circles, a term for fanatical soccer fans who are notorious for sometimes engaging in violence or associating with extremist right-wing political causes.

Mr. Volkov thanked the Polish and Lithuanian authorities for their work on his case in a post Friday on the social media platform X. “It is of enormous importance to investigate and to expose all the chain-of-command from Putin to the guy with the hammer,” he said.

The Insider, a Russia-focused investigative outlet, recently identified a man who was threatening a vocal anti-Kremlin Russian economist and his wife in Buenos Aires as a Polish citizen and shared that information with the Polish authorities, who subsequently arrested him, the outlet said in a report on Friday.

In the course of the investigation, the Polish authorities discovered that the man’s handler was also connected to the men who arrived in Lithuania last month to assault Mr. Volkov, The Insider reported.

The February killing of a Russian helicopter pilot in a seaside town in Spain months after his high-profile defection to Ukraine last summer also raised the specter of retribution on European soil against the Kremlin’s enemies. The perpetrators, still unknown, shot him six times in a parking garage before running over his body with a car.

Last year, the authorities in Britain arrested five citizens of Bulgaria and accused them of carrying out surveillance and information-gathering activities for Russian intelligence. The British authorities charged a sixth Bulgarian national in connection with the activities in February.

Tomas Dapkus contributed reporting from Vilnius, Lithuania, and Alina Lobzina from Berlin.