The campaign manager for the leading opposition candidate in Belarus has been detained on the eve of the presidential election, her office said.
A spokeswoman for candidate Svetlana Tikhanovskaya said Maria Moroz was expected to be held until Monday.
The government has not commented on the case, and it was not immediately clear on what grounds she was being held.
Alexander Lukashenko is seeking a sixth term in office in Sunday's vote. Large opposition rallies have been held.
Earlier on Saturday police briefly detained and then released another member of Ms Tikhanovskaya's team, Maria Kolesnikova. Police said she had been mistaken for another person, her office said.
The run-up to the election has seen the rise of 37-year-old Ms Tikhanovskaya and the biggest opposition protests for a decade.
A spokeswoman for Ms Tikhanovskaya previously said Ms Moroz was briefly detained on Thursday after visiting the Lithuanian embassy in Minsk. The interior ministry denied she had been arrested, telling AFP that the campaign manager had been "invited for a conversation".
Ms Moroz later said she was warned by the police not to organise unrest.
Stay-at-home mum Ms Tikhanovskaya is a political novice who only stepped in as a candidate for president when her husband was arrested and blocked from registering.
A second serious rival to Mr Lukashenko has also been jailed and a third has fled the country.
President Lukashenko, referred to by some as "Europe's last dictator", was first elected in 1994.