Prosecutors in Hamburg said the 31-year-old German citizen was the owner of a “National Bolshevist” Telegram account which openly supported the invasion of Ukraine.
They said he planned to travel to Belarus, a Russian ally, on August 18 and was suspected of breaking a German law that prevents soliciting for recruits on behalf of foreign forces.
On his social media accounts, which also included Facebook and the Russian website VK, he was alleged to have used the pro-Russian “Z” symbol on six occasions in an apparent show of support for the war.
The man also allegedly posed for a picture with an AK-47 rifle, a weapon of Soviet design, raising the suspicion that he was also in breach of weapons laws.
The AK-47 was not found when investigators searched his house in Hamburg on Thursday, but a number of knives were recovered along with pieces of computer hardware and other items.
Prosecutors did not say what they thought he planned to do in Minsk, but the country is a close Kremlin ally and under sanctions for helping Russian troops to wage war from its territory.
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They carried out the raid on his house after receiving a search and arrest warrant from a judge, a statement said.
Supporting crimes of aggression is an offence under German law. Russia's invasion has been widely condemned as a war of aggression aimed at seizing land and halting Ukraine's pro-western drift.
The Kremlin's claim that it is acting in self-defence, and in the defence of two unrecognised breakaway republics in eastern Ukraine, has won little acceptance.
More than half of the world's countries, 141 in total, supported a General Assembly resolution in March which said the war was a breach of the UN Charter. Only five voted with Russia.
German Chancellor Olaf Scholz has accused Russia of a further breach of international law over the apparent massacre of civilians in Bucha, near Kyiv, which he described as a war crime.