Belarus opposition figure Maria Kolesnikova tore up her passport at the border with Ukraine to avoid being forced from the country.
That was the claim of Anton Rodnenkov and Ivan Kravtsov, who say they were expelled by the country's security services.
But Rodnenkov and Kravtsov, also members of Belarus' opposition movement, said Kolesnikova resisted.
She (Kolesnikova) was forcibly pushed into the back seat, she shouted that she was not going anywhere," said Rodnenkov from a press conference in Ukraine on Tuesday.
"When she saw her passport, which was in the car, she immediately tore it and threw it out the window."
Kolesnikova, a member of the opposition coordination council created by the opposition to facilitate talks with long-time president Alexander Lukashenko about a transition of power, was detained on Monday in the capital of Minsk.
Colleagues hail 'smart move'
On Tuesday they were driven to the border, where authorities told them to cross into Ukraine. Kolesnikova refused and remained on the Belarusian side of the border in the custody of Belarusian authorities.
But Rodnenkov and Kravtsov crossed into Ukraine.
"Maria Kolesnikova is a hero. After 12 hours of interrogation - it is not clear where - she continued to demand justice, to demand a lawyer.
"I understand that she was then taken away by the special services.
"Where Maria was at the border she tore her passport so there was no legal possibility for her to go abroad. It was a very smart decision."
A spokesman for Belarus' Border Guard Committee, Anton Bychkovsky, confirmed that Kolesnikova was in the custody of Belarusian authorities but did not say what had happened at the border.
Belarus, in seeking to end a month of demonstrations since the presidential election on August 9, has used similar tactics to force other opposition figures out of the country.
They include Sviatlana Tsikhanouskaya, who under pressure, fled to neighbouring Lithuania. She told Euronews on Tuesday that Lukashenko had been seriously weakened by the ongoing demonstrations in the capital Minsk.
The main opposition challenger to Lukashenko repeated her call for international sanctions against the authoritarian leader and other government officials.
Police detain yet more protesters
Another associate of Tsikhanouskaya, Antonina Konovalova, went missing on Tuesday after being fined by a court for taking part in a weekend protest.
In the early evening, police moved in as several hundred demonstrators rallied in Minsk in solidarity with Kolesnikova. They dispersed several hundred people and detained at least a dozen.
The authorities have changed tactics since the mass arrests and brutal crackdown in the days following the vote, instead threatening and selectively detaining activists and protesters.
A criminal investigation has been opened into members of the opposition Coordination Council, who are accused of undermining national security. Two leading figures are now in Poland after facing threats recently.
The intimidation has failed to deter daily protests and the tens of thousands who have filled the streets at weekends.
Lukashenko claims majority support
In an interview with Russian journalists, Lukashenko insisted he retained the support of most of the country, saying it was "tragic" for him to face massive protests.
"I must protect what has been built with our hands, protect the people who have built it, and they are an overwhelming majority," he said.
The beleaguered leader has looked to Moscow for support. "If Belarus collapses today, Russia will come next," he added, blaming the United States for instigating the protests.
The Kremlin has endorsed Lukashenko's promises of reform, although he has only spoken vaguely about constitutional changes and a new election at an unspecified date.
For long tagged as "Europe's last dictator", has ruled the country for 26 years. He was declared the winner of August's election by a landslide but the vote is widely seen at home and abroad as having been rigged in his favour.