Moscow said on Friday it had detained a Russian citizen on treason charges for allegedly launching drones near military facilities to interfere with Russian air defence systems.
The announcement by the FSB security services came hours after the start of voting in presidential elections expected to hand Vladimir Putin another six-year term.
The FSB said the suspect had been carrying out tasks "aimed at countering Russian air defence systems," the FSB said.
"He assembled and launched unmanned aerial vehicles to create decoys in close proximity to Russian Defence Ministry facilities," it added.
The FSB said it had opened an investigation into high treason, a charge that can carry up to life in prison.
State media published footage showing masked men in military gear searching a small room, and arresting a man in his bed.
Asked who he was working for, the detainee answered: "the Freedom of Russia Legion," according to images published by the RIA Novosti news agency.
The Freedom of Russia Legion is a group made of pro-Kyiv Russian volunteers, and it has claimed several incursions into regions bordering Ukraine.
It launched a wave of guerilla-style attacks on border areas this week, claiming to have temporarily seized territory inside Russia.