Maria Kolesnikova 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 only decided to run in the vote after her husband was arrested and barred from standing. She was forced to leave Belarus for Lithuania the day after the vote, after she was detained for several hours.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.

Of the three women who joined forces in the election against the Belarus leader, only Maria Kolesnikova is still in the country.

Belarus reports 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, amidst 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.

Maria Kolesnikova is a prominent ally of Ms Tikhanovskaya, who left for Lithuania after her detention by authorities.

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 “sped off”, 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.

On Monday, eyewitnesses saw masked men seize Ms Kolesnikova on the street in central Minsk and push her into a minibus

“I’m the only one of the three of us who is still here,” Ms Kolesnikova said 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.”

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”.