Her office said the campaign manager for the leading opposition candidate in Belarus was arrested ahead of the presidential election.
Candidate Svetlana Tikhanovskaya spokesman said Maria Moroz is expected to be held until Monday.
The government has yet to comment on the case and it is not clear what she was detained for.
Alexander Lukashenko is looking for a sixth term in office in Sunday̵7;s vote. Major protests were held.
The eve of the election saw the rise of 37-year-old Tikhanovskaya and the biggest opposition protest in a decade.
A Tikhanovskaya spokesperson earlier said Moroz was briefly detained on Thursday after visiting the Lithuanian embassy in Minsk. The Interior Ministry denied she was arrested, telling AFP that the campaign manager had been “invited to speak”.
Ms. Moroz later said that she had been warned by the police not to hold the turmoil.
Mother Tikhanovskaya’s home is a novice politician who only participated in the presidential election when her husband was arrested and blocked from registration.
His second heavyweight rival Lukashenko has also been jailed and a third has fled the country.
President Lukashenko, called by some to be the “last dictator of Europe”, was elected for the first time in 1994.