Maria Kolesnikova says she was forced into a van on Monday by masked men in Minsk and traveled over 200 miles to the Ukrainian border.
The opposition leader was arrested at the border after destroying her passport in a no man’s land between Ukraine and Belarus the next day.
Ms Kolesnikova said in an official complaint released by her lawyer that agents from the Belarusian State Security Committee put a bag over her head before she fought for her deportation from Belarus and was imprisoned.
“They threatened to kill me,” Ms. Kolesnikova said. “They said that if I refused to voluntarily leave the territory of Belarus, they would take me out of the country anyway – alive or in fragments. ”
The 38-year-old is being held in Minsk for endangering national security.
Ms Kolesnikova is part of a group of opposition members protesting President Alexander Lukashenko’s authoritarian leadership.
This week Belarus marked a month since massive protests erupted against the re-election of Mr Lukashenko.
The opposition rejects the country’s vote in August as rigged, and protesters demanding Lukashenko’s resignation represent an unprecedented challenge to the Belarus leader’s 26-year reign.
Ms Kolesnikova is a prominent member of the Coordination Council set up by opposition activists to lobby for a new election.
Police raided the headquarters of protest groups on Wednesday to arrest several activists.
Lawyer Maxim Znak has been arrested, leaving Nobel Prize-winning writer Svetlana Alexievich as one of the few high-level activists still free and currently still in Belarus.
She said she received threatening phone calls and feared she would be the state’s next target.
Earlier Thursday, a group of men wearing masks and holding crowbars broke into the headquarters of Viktar Babaryka, a political rival of Alexander Lukashenko, according to CCTV obtained by Sky News.