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”.
“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 cafe.
French President Emmanuel Macron said the man was not working for France.
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”.
Later on Thursday, President Macron said the detained man was “in no way someone who was working for France”.
Relations between Paris and Moscow are strained over Russia’s full-scale invasion of Ukraine in 2022.
In a statement on Thursday, the SK said a criminal case had been opened, and the suspect would be charged in the “nearest future”.
The Frenchman, it said, had repeatedly visited Russia for several years, holding meetings with Russian nationals.
The 19-second video released by the SK shows a man, whose face is blurred, being detained by Russian law enforcement officers, before being led away to an SK van.
The Frenchman is also accused of failing to submit documents needed for “inclusion in the register of foreign agents”.
Russia requires anyone who gets foreign support or is under influence from abroad to declare themselves such an agent. It has used the 2012 law to squash any opposition inside the country to President Vladimir Putin, and also as a pretext to detain people, including foreigners.
Last year, Russian-American journalist Alsu Kurmasheva was arrested on a similar charge.
The charge carries a jail term of up to five years.