Georgia's main opposition leader Gigi Ugulava has been arrested while boarding a flight to Ukraine.
Mr Ugulava, a close ally of ex-President Mikheil Saakashvili, is facing allegations of money laundering in the funding of his party's campaign in June municipal elections.
Before being taken away, he said he had not been trying to flee and had fully intended to return the same day.
His supporters say his arrest is politically motivated.
Mr Ugalava's United National Movement (UNM) party issued a statement, saying his detention was timed to coincide with the campaign for the second round run-off of the elections.
It was a "deepening of the government's agenda of political retribution and persecution of the opposition," the statement read.
But Interior Ministry Aleksandre Chikaidze said Mr Ugulava had been warned not to leave Georgia because he had been accused of embezzling government funds.
The UNM was founded by Mr Saakashvili, whose presidency was overshadowed by a brief conflict with Russia in 2008.
In a separate development on Thursday, the European Court of Human Rights condemned Russia's treatment of thousands of Georgians in the run-up to the war.
As relations between the two countries grew strained in late 2006, Georgia said 2,300 of its citizens were arrested and forcibly expelled by the Russian authorities and another 2,300 had to leave by themselves.
Relations are still tense between the two governments, although ties have improved since the new government came to power in October 2012.
However, Georgia, along with Ukraine and Moldova, signed a European Association Agreement with the EU last week deepening trade relations with the 28-member bloc.
Russia responded to the deal on Thursday by limiting meat imports from Moldova, a measure seen by the European Union as a political decision. Moldova's parliament ratified the EU deal on the same day.