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.
State media say Maria Kolesnikova was detained while trying to cross into Ukraine in the early hours of Tuesday.
But Ukrainian reports say she tore up her passport to prevent expulsion as two colleagues were forced to leave.
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 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.
and says it is planning to impose sanctions.
Mr Lukashenko has ruled his country since 1994. He has accused Western powers of interference and has been supported by President Vladimir Putin of Russia. He 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, opposition leader Svetlana 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 Belarus for Lithuania the day after the 9 August vote following 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, where Ms Kolesnikova now is.
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. She was now in detention, he added, while the two others were in Ukraine.
Ukraine has 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 is citing “informed sources”.
On Monday, eyewitnesses saw masked men seize Ms Kolesnikova on the street in central Belarus 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. It added that press secretary Mr Rodnenkov and executive secretary Mr Kravtsov had also disappeared.
The interior ministry said it had no information about any of the council members being detained.
Who are the three women?
Ms Kolesnikova is the last of the three women who joined forces against Mr Lukashenko to remain inside Belarus.
She was initially the campaign manager for presidential candidate Viktor Barbaryko before his arrest in June, when she decided to work with Veronika Tsepkalo and Svetlana Tikhanovskaya,
Ms Tikhanovskaya stood against Mr Lukashenko in the election on 9 August. She only decided to run after her husband was arrested and barred from standing, and is now in Lithuania after she was forced to leave Belarus following the vote.
Ms Tsepkalo has travelled to Poland with her husband Valery and children. Mr Tsepkalo, the former ambassador to the US for Belarus, was also barred from standing against President Lukashenko.
Another female activist, Olga Kovalkova, announced on Saturday she had fled to Poland amid threats of imprisonment.
“I’m the only one of the three of us who is still here,” Ms Kolesnikova told BBC Russian in an interview last month. “To understand exactly what’s going on, you really have to be here.”
Ms Kolesnikova described the recent demonstrations as “not a struggle for power” but “a struggle for human dignity and self-respect”. She said she and her team had decided against using bodyguards.
“No number of guards would be of any use if a bus full of riot police stopped us,” she said. “We all know what a police state is capable of.”