Germany's Foreign Office on Wednesday indirectly confirmed the detention of a German citizen in the Russian city of St Petersburg.
A spokesman said in response to a journalist's question that the case was known to him and that efforts were being made to provide consular assistance.
"However, we do not comment on individual consular cases of this nature where we are assisting German nationals," he added.
Citing Russian media reports, Germany's Welt and Der Spiegel had reported that a 41-year-old man had been detained after donating around 900 roubles ($10) to the organization of Alexei Navalny, the Kremlin critic who died in a penal colony.
According to the reports, the man - who is said to run a gym in St Petersburg - was accused of transferring money to an extremist organization.
The Foreign Office spokesman said: "Ever since the start of Russia's war of aggression against Ukraine, at the very latest, any form of resistance has been dealt with mercilessly. The issue here is that money is alleged to have been donated to the Navalny organization. This fits into that picture."
"This is not a trend; it is the reality of everyday life in Russia," he added.