She was one of three women who joined forces ahead of August's presidential election to challenge incumbent Alexander Lukashenko
Masked men have apparently detained a leading opposition figure in Belarus.
Witnesses saw Maria Kolesnikova being bundled into a minibus in Minsk and driven away, reports the BBC citing local media report.
She was one of three women who joined forces ahead of August's presidential election to challenge incumbent Alexander Lukashenko.
Mass unrest has followed his re-election amid allegations of vote-rigging. Hundreds more people were arrested on Sunday.
At least four people have died and hundreds have been injured as authorities have tried to crush dissent in the country. EU leaders do not recognise the results of the election and have agreed to impose sanctions on Belarus.
But Lukashenko - who has been in power since 1994 - has blamed Western nations for interfering in his country. On Monday, the Kremlin announced he would visit Moscow for talks "in the coming days". Russia is a close ally of Lukashenko's.
An eyewitness told Belarus news outlet Tut.by that she saw masked men take Kolesnikova's mobile phone and push her into a minibus on Monday morning.
Police in the capital, Minsk, have not yet commented on the reports.
Kolesnikova was a member of the Co-ordination Council set up by the opposition to ensure a transfer of power. Government authorities have launched a criminal case against opposition leaders, saying the "creation and activity of the Co-ordination Council are aimed at the seizure of state power, and at harming national security".