A Russian pro-democracy activist residing in Serbia and a fierce critic of the invasion of Ukraine said Thursday that Serbian authorities have banned him from entering the country upon returning from a trip abroad.

Peter Nikitin told The Associated Press he was stopped at Belgrade airport's passport control early on Thursday, returning from a holiday in Portugal. He said the decision was "arbitrary and illegal."

Though it formally seeks European Union membership and has condemned Russia's invasion of Ukraine, Serbia has maintained friendly relations with Moscow and refused to impose Western-backed sanctions over the aggression.

The United States this week imposed sanctions on Serbia’s pro-Russian intelligence chief, Aleksandar Vulin, accusing him of involvement in illegal arms shipments, drug trafficking and misuse of public office that help "facilitate Russian malign activities in Serbia and the region."

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Nikitin holds both Russian and Dutch citizenship and has a Serbian residence permit, where he and his family have lived for years. He said he was trying to enter Serbia using his Dutch passport when he was turned back and ordered to return to Frankfurt, Germany, from where he had flown in.

"I have been in the border zone at the Nikola Tesla airport (in Belgrade) the entire night with no sleep," said Nikitin. "I will stay here until they let me go home or deport me forcefully."

Nikitin sent the AP a document border authorities gave him which read "protective measure of removal, the security measure of expulsion or ban on entry into the Republic of Serbia in effect."

Serbian authorities have issued no immediate statement.

Peter Nikitin

Peter Nikitin, a Russian pro-democracy activist residing in Serbia, shouts slogans during a protest against Russia's foreign minister in Belgrade, Serbia, on June 6, 2022.  (AP Photo/Darko Vojinovic)

"The document (received) has no explanation for the ban except that it is in force against me," Nikitin said. "I will appeal this arbitrary and illegal decision."

Serbian lawyer, Cedomir Stojkovic, who has teamed up with Nikitin in the past in opposing Russia's influence in Serbia, said expelling or banning a permanent resident card holder from entering the country can only be ordered by a court of law.

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"No court procedure has ever been initiated against Nikitin and therefore no court decision about his expulsion could have been made," Stojkovic said in a statement.

Nikitin is well known as an outspoken critic of Russia's invasion of Ukraine and the regime of Russian President Vladimir Putin. He was one of the organizers of antiwar and pro-democracy protests in Serbia called for by Russians and Ukrainians living in the country.

Some 200,000 Russian citizens have moved to Serbia since the start of the war in Ukraine as the Balkan country requires no entry visas for Russians and is a fellow-Slavic nation. Many have fled being drafted into the army or moved their businesses to a sanctions-free country.

In January, Nikitin and Stojkovic together filed a criminal complaint against Vulin and several others for allegedly allowing the recruitment of Serbs to fight for Wagner mercenaries in Ukraine, at the time fighting on Russia's behalf.

In 2021, Vulin created a "working group" with Nikolai Patrushev, the powerful secretary of the Kremlin’s Security Council, to fight "color revolutions" — a series of mass protests that occasionally led to the toppling of regimes in the former Soviet Union, the former Yugoslavia, the Middle East and Asia.

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Serbian independent media reported that at their meeting in Moscow in late 2021, Vulin gave Patrushev wiretaps from a Belgrade meeting held by members of the Russian opposition. Shortly afterward, some of those who attended the meeting were arrested in Russia.

Vulin has denied that he handed over the tapes to Patrushev.