BELGRADE, Serbia (AP) — 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, Serbia has maintained friendly relations with Russia and refused to impose Western-backed sanctions against Moscow over the invasion of Ukraine.

Nikitin holds both Russian and Dutch citizenship and has a Serbian residence permit, where he has 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.

“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.”

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.

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.