The Interfax Ukraine news agency said the activist had deliberately ripped up her passport so that Ukrainian border officials would not be able to let her through.

Deputy Ukrainian Interior Minister Anton Gerashchenko said Kolesnikova, who had been missing for the previous 24 hours, had successfully prevented "a forcible expulsion from her native country".

Soon after the incident, President Alexander Lukashenko told Russian reporters that he would not rule out holding new elections after four weeks of mass protests, but rejected any talks with opponents who he said were bent on steering the country towards disaster.

The fate of Kolesnikova, a key figure during the protests, had been a mystery since supporters said she was snatched in the street by masked men in the capital Minsk on Monday.

"Maria Kolesnikova made a courageous act that did not allow the Belarusian special services to expel her to the territory of Ukraine," Gerashchenko told Reuters.

"All responsibility for her life and health is on Alexander Lukashenko, the dictator of Belarus."

Kolesnikova's whereabouts were unclear.

"Kolesnikova has now been detained, I can't say concretely where she is, but she has been detained," Anton Bychkovsky, a representative of the Belarusian border service, told Reuters.

Lukashenko, who has crushed dissent in Belarus for the past 26 years, conceded in his interview with Russian journalists that he may have held onto power too long.

"Yes, maybe I overstayed a bit," Russian journalist Roman Babayan cited Lukashenko as saying.

But the 66-year-old leader said he was the only person capable of protecting the country for now.

Lukashenko said his supporters would be attacked if he left power, said Babayan, editor-in-chief of the Moscow Talks radio station.

"I won't talk to the opposition Coordination Council because I don't know who these people are. They are no sort of opposition. Everything they suggest is a catastrophe for Belarus and the Belarusian people," he said.

Belarus has seen a tense political stalemate since a disputed presidential election on August 9, with protesters flooding the streets and security forces responding with mass arrests and - according to some of those detained - beatings and torture.

The country of 9.5 million people is of key strategic interest to Moscow as a route for its oil and gas exports and a buffer between NATO and Russia.

Lukashenko denies opponents' accusations of vote-rigging and has accused foreign powers of trying to topple him in a revolution. He retains the backing of Russian President Vladimir Putin, a key ally, but faces the prospect of European Union sanctions later this month against Belarusian officials involved in the election and its violent aftermath.

Sviatlana Tsikhanouskaya, who stood against him in the August election, said on Tuesday he was heading an "illegitimate regime".

"Lukashenko does not have any legitimacy as the president of our country. He does not represent Belarus anymore," she told a Council of Europe committee from her headquarters in Vilnius, Lithuania, where she fled after the election.

Australian Associated Press