Prominent opposition leader Maria Kolesnikova was detained by Belarusian authorities, a border official said, after thwarting what a Ukrainian government minister described as an attempt to expel her from Belarus.
The Interfax Ukraine news agency said the activist had deliberately ripped up her passport so that Ukrainian border officials would not be able to let her through.
Deputy Ukrainian Interior Minister Anton Gerashchenko said on Facebook that Ms Kolesnikova, who had been missing for 24 hours, had successfully prevented "a forcible expulsion from her native country".
The fate of Ms Kolesnikova, a key figure in weeks of mass protests over the disputed re-election of Belarusian President Alexander Lukashenko, had been a mystery since supporters said she was snatched in the street by masked men in the capital Minsk on Monday.
"Maria Kolesnikova was not able to be deported from Belarus as this brave woman took steps to prevent herself from being moved across the border.
"She remained on the territory of the republic of Belarus. Alexander Lukashenko is personally responsible for her life and health," Mr Gerashchenko said.
Ms Kolesnikova's whereabouts are unclear. Two other opposition figures who had gone missing around the same time as her, Anton Rodnenkov and Ivan Kravtsov, did enter Ukraine in the early hours of yesterday morning, the Ukrainian border service said.
"Kolesnikova has now been detained, I can't say concretely where she is, but she has been detained," Anton Bychkovsky, a representative of the Belarusian border service, said.
"She was detained in connection with the circumstances under which they (the group) left the territory of Belarus."
With the detention of Ms Kolesnikova and the arrest or flight of other opposition figures, Mr Lukashenko has attempted to cripple the leadership of the protest movement, yet it shows no sign of abating after four weeks of mass demonstrations and strikes.
Svetlana Tikhanouskaya, who stood against him in the presidential election last month which the opposition claim was rigged, said yesterday that he was heading an "illegitimate regime".
"Lukashenko does not have any legitimacy as the president of our country. He does not represent Belarus anymore," she told a Council of Europe committee from her headquarters in Vilnius, Lithuania, where she fled after the election.