A day after a Belarus opposition leader was seen being bundled into a van by masked men, she has been involved in dramatic scenes at the border.
Belarus says Maria Kolesnikova was detained while trying to cross into Ukraine in the early hours of Tuesday.
But Ukraine says Belarusian officials tried to expel her, amid reports that she tore up her passport to stop them.
She is one of three women who joined forces to challenge President Alexander Lukashenko in August's election.
Mass protests erupted after election authorities awarded Mr Lukashenko victory amid allegations of vote-rigging. The main opposition figure, Svetlana Tikhanovskaya, says she won 60-70% of the vote, where votes were properly counted.
The latest events on the border with Ukraine come after a fourth weekend of anti-government demonstrations. Authorities said more than 600 people were arrested on Sunday. The EU has demanded the release of all political prisoners and says it is planning to impose sanctions.
Mr Lukashenko, who has ruled his country since 1994, admitted on Tuesday that perhaps he had remained in power too long: "Yes, perhaps I've overstayed a bit, perhaps," he was quoted as saying by Russian reporters, before making clear he would not leave "just like that".
He has accused Western powers of interference and is expected to visit Moscow "in the coming days" amid claims by Lithuania that he is planning deeper integration with Russia.
In an address to a Council of Europe committee, Ms Tikhanovskaya warned that any treaty made by Mr Lukashenko's "illegitimate regime" would not be upheld "by a democratically elected Belarus government".
Maria Kolesnikova is a prominent ally of Ms Tikhanovskaya, who left for Lithuania after her detention by authorities.
What's happened to Ms Kolesnikova?There are conflicting reports about what happened at around 05:00 (02:00 GMT) on Tuesday at a border crossing south of the city of Gomel.
According to Belarus officials, she was with two opposition colleagues, Anton Rodnenkov and Ivan Kravtsov, who were in a BMW car. At the crossing, the car "accelerated sharply", and Ms Kolesnikova "found herself outside the vehicle". The official said she was "pushed out of it" and it continued to move towards Ukraine. Mr Lukashenko told Russian reporters she had been held for "violating the rules on crossing the state border".
But Ukraine and opposition activists have rejected Belarus's version of events. Anton Geraschenko, Ukraine's deputy internal affairs minister, described the two men's departure as "forcible expulsion".
"Maria Kolesnikova could not be expelled from Belarus, because this brave woman took action to prevent her movement across the border," he wrote in a Facebook post. "She remained on the territory of the Republic of Belarus."
She is then reported to have torn up her passport at the border so she could not enter Ukraine, according to Interfax-Ukraine news agency which cited "informed sources".
On Monday, eyewitnesses saw masked men seize Ms Kolesnikova on the street in central Minsk and push her into a minibus.
The Co-ordination Council - a body set up by the opposition to oversee a transfer of power after the disputed election - later said it had no idea of her whereabouts.