Belarusian opposition figure Maria Kolesnikova will file a complaint against members of the security service for "kidnapping" and "death threats", her political organisation has announced.
Kolesnikova is currently being held by the country's Investigative Committee under investigation for "causing harm to national security".
Opposition colleagues claim she tore her passport at the border with Ukraine to avoid being exiled out of Belarus.
Kolesnikova's lawyer, Lyudmila Kazak, has submitted a complaint to the Investigative Committee on Thursday to initiate a criminal case.
A statement was released on the website of Viktor Babaryko, a former presidential candidate for whom Kolesnikova was the campaign manager, detailing her movements before the Ukrainian border.
Extracts in the letter refer to "abduction by unknown people, psychological pressure, death threats, [and] an attempt to force her out of Belarus."
The website also claims that Maria Kolesnikova is ready to give "detailed testimony" against members of the KGB and GUBOPiK in Belarus.
"These people threatened to take my life, which I perceived as real," wrote Kolesnikova.
"In particular, it was stated that if I did not leave the territory of the Republic of Belarus voluntarily, I would still be taken away: alive or in parts."
"There were also threats to deprive me of my liberty for up to 25 years, to cause me problems in places of detention, in places of deprivation of liberty, which I also perceived as real."
Kolesnikova claims that when she did not consent to the authorities, she was kept in a cell for one hour, the letter continues.
She also confirms the version of her supporters that, in order not to be forcibly expelled from the country, she tore up her passport.
"After the KGB officers realized that I would not leave Belarus voluntarily, they put a bag on my head, pushed me into a minibus, and took me to the [Aleksandrovka village], where they tried to use force against my will to expel me from the Republic of Belarus."
"After I tore my passport, thus excluding entry into the territory of Ukraine, I was again put in a minibus and taken to the Mozyr border detachment, where I was until the evening of September 8, 2020."
The letter also states that Kolesnikova was "warned about criminal liability for knowingly false denunciation".
Her detention came one month after the disputed presidential election result, which triggered an unprecedented wave of protest in the country.
President Alexander Lukashenko, who has been in power since 1994, stated on Thursday that he will not leave under pressure.
Lyudmila Kazak told Euronews that Kolesnikova was interrogated on Thursday as a suspect of an "illegal seizure of power", which carries a 26-year prison sentence in Belarus.
"She is not officially accused," said Kazak, "she could be detained for 10 days before the official accusation according to the law."
Kolesnikova's lawyer also told Euronews that the opposition figure was "looking good" and "very motivated", and is currently sharing a cell with five other women.
Concerns remain for the remaining members of the opposition co-ordination council in Belarus, including former Nobel laureate Svetlana Alexievich, who was visited by European diplomats when she reported that masked men had tried to break into her flat on Wednesday.
"We are all standing behind Maria," another free member of the council, Max Bogretsov, told Euronews on Thursday.
"What we are accused of - seizure of power - was never part of the agenda, we never did anything that has been incriminated by authorities".
"We're pretty sure that whatever happens to us, new people will join the movement and it is going to move forward," said Bogretsov.
"The more violence that authorities apply and the more pressure that authorities apply, the larger the movement is going to get."
"That's our belief and we're going to stand by it"