Moscow says it has detained a French citizen accused of gathering information about Russia's military activities and failing to register as a "foreign agent", reports BBC News.
"Such information, if obtained by foreign sources, may be used against the security of the state," said Russia's Investigative Committee (SK).
It also posted footage purportedly showing the arrest of the unnamed male suspect in a Moscow café.
A Switzerland-based NGO later said its adviser Laurent Vinatier was held in the Russian capital.
"We are aware that Laurent Vinatier, an adviser at the Centre for Humanitarian Dialogue (HD), has been detained in Russia," a spokesman for the non-government organisation told the AFP news agency.
"We are working to get more details of the circumstances and to secure Laurent's release," he added.
The NGO states its main activity is "to prevent and resolve armed conflicts around the world through mediation and discreet diplomacy".
France made no immediate comments on the issue. Ties between Paris and Moscow are strained over Russia's full-scale invasion of Ukraine in 2022.