Estonia's consul to Saint Petersburg has been detained for activities "incompatible with the status of a diplomatic worker" and "openly hostile" to Russia, the FSB security service said.
Russian security forces have said they had detained Estonia's consul to Saint Petersburg for allegedly receiving classified documents, the latest in a series of espionage cases between Moscow and the West.
The FSB security service said consul Mart Latte was "caught red-handed while receiving classified documents from a Russian national" and detained for activities "incompatible with the status of a diplomatic worker" and "openly hostile" to Russia.
"Measures will be taken against the foreign diplomat in accordance with the rules of international law," said the statement.
No further details were provided.
Aari Lemmik, spokesperson for Estonia's Foreign Ministry, said the FSB held Latte for an hour-and-a-half and charged him with "embezzling sensitive materials."
She told AFP that the charges were "completely ungrounded" and the diplomat's arrest was "a provocation by Russian authorities."
"This is yet another example that Russia is not interested in constructive relations with its neighbours and the European Union," she said.
The Baltic states of Estonia, Latvia and Lithuania, which were ruled by Moscow during Soviet times, are EU and NATO members and among the fiercest international critics of Moscow.
Russia and Western countries have engaged in a series of diplomatic expulsions and mutual espionage claims as tensions spiralled since the start of the year.
Moscow in April expelled four diplomats from the Baltic states in a tit-for-tat response to a wave of expulsions of Russian diplomats in central and eastern Europe.
Those expulsions were made in solidarity with the Czech Republic, which expelled 18 Russian diplomats after accusing Moscow's secret services of being behind a fatal explosion at an arms depot in the country's east in 2014.
Recent months have also seen German police arrest a Russian scientist working at a local university for alleged spying on behalf of Moscow, and the arrest of an Italian navy captain caught red-handed by police while selling confidential military documents to a Russian embassy official.