Olga Kovalkova, a confidant of the main Belarusian opposition candidate Svetlana Tikhanovskaya, was removed from the country on Saturday night, according to a statement from the organization.
The council is a body aimed at coordinating a peaceful and orderly transition of power and was established by Tikhanovskaya following Belarus’ disputed August elections.
“On the night of September 5, the Belarusian special services took Olga Kovalkova, a confidant of Svetlana Tikhanovskaya, to the Belarusian-Polish border and released her to a no-man’s land,” the statement said.
According to the council, Kovalkova was offered two choices: She could be taken out of the country or she would be kept in custody in Belarus, with further terms of imprisonment constantly added to her sentence.
Kovalkova was then masked by security service members, placed in the back of a car and driven to the Belarus border town of Bruzgi, the statement added.
Once the activist was freed at the Polish side of the border, she boarded a regular bus to Warsaw.
Tikhanovskaya is also in exile, having fled to the Baltic nation of Lithuania at the height of the protest movement against President Alexander Lukahsenko.
Tikhanovskaya was the opposition candidate in Belarus’ August 9 election in which Lukashenko claimed a contested victory. Independent observers have criticized the August vote for being neither free nor fair. Unrest erupted soon after the result emerged, with tens of thousands marching in Minsk, the nation’s capital.
Thousands also marched in Minsk Sunday, as the anti-government protests continued.
More than 135 people were detained as security services cracked down on the mass demonstration, according to local human rights watchdog Viasna 96. The rights group said multiple arrests were made in other cities across Belarus, including Grodno and Brest.
The Belarus Interior Ministry has not yet responded to CNN’s request for comment.
The demonstration marks the fourth consecutive weekend of mass opposition protests in Belarus.
Lukashenko is often described as Europe’s last dictator and has remained defiant in the face of unrest.
On September 2 Belarusian police officers instigated a harsh crackdown on student protesters, after thousands marched in Minsk’s Independence Square against the government.
Correction: An earlier version of this story misspelled the last name of Belarusian opposition figure Olga Kovalkova.