Prominent Russian opposition politician Andrei Pivovarov says he was arrested on board a plane as it was leaving for Poland.
Police officers stopped the plane, which was already taxiing, in St Petersburg on Monday evening, Pivovarov wrote on his Telegram channel.
According to the 39-year-old, the authorities' action is connected to his former activity as chairman of the organisation Open Russia, which was critical of the Kremlin.
The relevant investigative agency said on Tuesday that he is accused of being involved with an "undesirable organisation," a designation that was given to Open Russia a few days ago, effectively banning it.
Pivovarov faces up to six years in prison if found guilty.
After Open Russia, which is supported by Mikhail Khodorkovsky, a powerful critic of President Vladimir Putin, was declared an "undesirable organisation," Pivovarov announced he would disband it.
The fact that he was still detained anyway was a sign of "complete lawlessness," Pivovarov said.
The politician was to be taken to the city of Krasnodar about 1200 kilometres south of Moscow on Tuesday, the state agency TASS reported, citing security forces.
Pivovarov's lawyer explained the transfer with several Facebook posts that her client had published from Krasnodar about a year ago.
In one of them, he had expressed solidarity with an opposition politician who was being persecuted by the Russian judiciary.
The European Union's top diplomat Josep Borrell called on Russian authorities "to immediately and unconditionally release Mr Pivovarov".
"This case is not an isolated incident but confirms a continuous pattern of shrinking space for civil society, the opposition and critical voices as well as independent media," he said in a statement.
On Tuesday, the Moscow opposition activist Dmitry Gudkov was also detained.
He had reported earlier that his dacha had been searched.
The flat of an employee was also affected, he wrote on Telegram.