Belarusian opposition leader Maria Kolesnikova has travelled to Ukraine with two others, Belarusian border guards said on Tuesday.
38-year-old Kolesnikova is one of the prominent opposition members in Belarus who oppose the controversial head of state Lukashenko.
She was picked up by masked men in the centre of Minsk on Monday and taken away in a minibus. Kolesnikova repeatedly showed up at the protests and was cheered on by the protesters.
Several countries have expressed concern about the disappearance of the opposition leader.
The countries have asked Belarusian President Alexandr Lukashenko for clarification and demanded her immediate release if detained by the authorities.