Carrying just her passport, money and a few essentials in her handbag, Arina Malinovskaya fled for the Ukrainian capital Kyiv.
The decision to boycott follows the forced diversion of a plane on Sunday by Belarusian authorities.
Pavel Seviarynets was sentenced along with six other activists on Tuesday amid international fury over Minsk's forced diversion of a passenger jet to arrest an opposition journalist.
A 48-year-old Dutch citizen was detained by authorities on Sunday after trying to cycle into Belarus from neighbouring Lithuania.
Sviatlana Tsikhanouskaya has been the face of Belarus' opposition. But two new political movements could threaten that, fracturing those who want President Alexander Lukashenko to step aside.