Russia has declared opposition leader Alexei Navalny's Anti-Corruption Foundation a "foreign agent".
The move by the justice ministry means the organisation will now be subject to more checks by the authorities.
It will also have to clearly state its "foreign agent" status on all official documents, and could be suspended.
Mr Navalny, who has led nationwide protests against President Vladimir Putin's rule, denied his foundation had received "foreign money".
The 43-year-old opposition leader, who has been arrested a number of times, describes the governing United Russia party as a group of "crooks and thieves".
His candidacy in the 2018 presidential election was banned by authorities over his conviction by a Russian court of embezzlement, which bars him from running for office.
Mr Navalny vehemently denies the accusations, saying his legal troubles are Kremlin reprisals for his fierce criticism.
In a short statement, the Russian justice ministry said the decision to classify Mr Navalny's non-commercial foundation as a "foreign agent" was taken following an audit.
It provided no further details.
In August, a money-laundering investigation was launched into Mr Navalny's foundation, amid big street protests in Moscow against the exclusion of opposition candidates from elections to the city parliament.
Last month, dozens of Mr Navalny's regional offices were raided.
What is the 'foreign agent' law?
Introduced in 2012, it requires non-profit organisations, charities and civil society groups that have foreign funding and engage in political activity in Russia to declare themselves as "foreign agents".
They then automatically become subject to additional requirements, and failure to meet them could result in the suspension of their activities.
Opponents accuse the Russian authorities of trying to suppress its critics and curb basic freedoms in the country.
The Kremlin denies this, saying the law prevents foreign organisations from interfering in Russia's internal affairs.