Girkin, also known as Igor Strelkov, helped Russia annex Crimea from Ukraine in 2014 and then organise pro-Russian militias who took control of part of eastern Ukraine from Kyiv.
Girkin's lawyer told the state news agency TASS that it was not clear why his client had been detained.
RBC, citing two unnamed law enforcement sources, said Girkin's Moscow home was being searched and that he had been detained over a complaint against him made by a former Wagner employee.
Tatiana Stanovaya, founder of the R.Politik analysis firm, said the men who run Russia's law enforcement and power ministries had long wanted to arrest Girkin.
Stanovaya said Girkin's detention was a signal that any of the bitterest critics of Moscow's approach to the war could face prosecution.
Persons:
Putin, Igor Girkin, Vladimir Putin, Yevgeny Prigozhin, Wagner, Igor Strelkov, Girkin, PUTIN, Miroslava Reginskaya, Girkin's, Tatiana Stanovaya, Stanovaya, Andrew Osborn, Kevin Liffey
Organizations:
RBC, Malaysia Airlines, Federal Security Service, of Angry Patriots, Kremlin, Telegram, Investigative, TASS, Thomson
Locations:
Russia, Ukraine, MOSCOW, Russian, Crimea, Kyiv, Girkin's Moscow