Maria Kolesnikova, a member of the Co-ordination Council created by the opposition to facilitate talks with longtime leader President Alexander Lukashenko on a transition of power, was detained on Monday in the capital Minsk along with two other council members.
Early on Tuesday, they were driven to the Ukrainian border, where the authorities told them to cross into Ukraine.
Ms Kolesnikova refused and remained on the Belarusian side of the border in the custody of the Belarusian authorities.
Two other council members, Ivan Kravtsov and Anton Rodnenkov, crossed into Ukraine.
Anton Bychkovsky, spokesman for Belarus’s Border Guard Committee, confirmed that Ms Kolesnikova is in the custody of Belarusian authorities.
Mr Lukashenko, 66, has ruled the country for 26 years, relentlessly stifling dissent and keeping most of the economy in state hands.
Belarus has applied similar tactics to other opposition figures, seeking to end a month of demonstrations against the re-election of Mr Lukashenko in a vote the protesters see as rigged.
Sviatlana Tsikhanouskaya, the main opposition challenger to Mr Lukashenko, left for Lithuania a day after the August 9 vote, under pressure from the authorities.
On Saturday, a top associate of Ms Tsikhanouskaya, Olga Kovalkova, moved to Poland after the authorities threatened to keep her in jail for a long time if she refused to leave the country.
Ms Kovalkova said agents of the Belarusian State Security Committee put her into a car, where she was told to lay on the floor, unaware of where they were taking her.
She was dropped off in no man’s land between the Belarus and Poland border, and Polish border guards asked a bus driver driving into Poland to take her on board.
Belarusian prosecutors have opened a criminal probe against members of the Co-ordination Council that opposition activists set up after the election to try to negotiate a transition of power.
Several council members were arrested and some others called for questioning.
Last week, Ms Kolesnikova announced the creation of a new party, Together.
She said the move will help overcome the current crisis, but the party founders’ call for constitutional changes has stunned some other opposition council members, who argued that it could divert attention from the main goal of getting Mr Lukashenko to step down.
