Russian authorities have charged an Estonian security officer arrested last week with spying.
Eston Kohver could face up to 20 years in jail if convicted, according to his lawyers.
Prosecutors say he was carrying concealed video and audio recording equipment, as well as a handgun and some 5,000 euros in cash.
Estonia claims he was abducted at gunpoint on the border and taken to Moscow as he tried to stop illegal activity involving the smuggling of goods, weapons and migrants.
Relations between Russia and Estonia have deteriorated in recent months because of Moscow’s annexation of Crimea and alleged support for pro-Russian separatists.