Her office said the campaign manager of the main opposition candidate in Belarus was detained on the eve of the presidential election.
A spokeswoman for candidate Svetlana Tikhanovskaya said that Maria Moroz is expected to be held until Monday.
The government has not yet commented on the case, and it is unclear why she was detained.
Alexander Lukashenko hopes to seek a sixth term in the ballot on Sunday. A large-scale opposition rally was held.
On the eve of the election, the 37-year-old Ms. Tikhanovskaya rose up, the biggest opposition protest in a decade.
A spokesperson for Ms. Tikanovskaya previously stated that Ms. Moroz was briefly detained on Thursday after visiting the Lithuanian Embassy in Minsk. The Ministry of the Interior denied that she had been arrested and told AFP that the campaign manager was “invited for dialogue.”
Ms. Moroz later said that the police warned her not to organize unrest.
Ms. Tikhanovskaya, the mother who stayed at home, was a political novice and only became a presidential candidate when her husband was arrested and prohibited from registering.
Lukashenko’s second serious competitor has also been in jail, and one third fled the country.
President Lukashenko (someone called “Europe’s last dictator”) was first elected in 1994.