Maria Kolesnikova, a member of the Coordination Council created by the opposition to facilitate talks with President Alexander Lukashenko on a transition of power, was reportedly put on a minibus in the capital, Minsk, and driven away by unidentified people.
Her disappearance follows a massive rally Sunday in Minsk that drew an estimated 100,000 protesters demanding the resignation of Lukashenko, who extended his 26-year rule in an August 9 election that the opposition and some poll workers say was rigged. They are small-scale in the regions.
Belarusians have been demonstrating across the country for almost a month even though the protest movement lacks a clear leader, with many activists jailed or forced out of the country. At least four people are reported to have died during the demonstrations.More news: Kamala Harris Blasts Trump For Denying Systemic Racism In U.S.
The protests are unprecedented in Belarus, which has been under Lukashenko's rule since 1994.
As the rain fell down, they converged together from different districts of the Belarusian capital in the city centre, initially heading for Lukashenka's residence - the Independence Palace.
Lukashenko, clinging on to power, has taken to appearing in public toting an automatic weapon and issuing dire warnings that North Atlantic Treaty Organisation is about to invade, comments rejected by North Atlantic Treaty Organisation leaders.
Students wore red and white opposition colors as they protested Lukashenko for the fourth week across the city.
The European Union has already slapped sanctions on the Belarus Government in response to what it sees as a sham election result.More news: United Kingdom warned over Brexit deal commitments
"Protesters want the long-time president to resign after his re-election last month amid allegations of ballot-rigging".
Kolesnikova is the last of three female politicians left inside Belarus who joined forces before an August 9 presidential election to try to challenge veteran incumbent Alexander Lukashenko.
Her alleged detainment comes after authorities in Belarus launched a tough crackdown on protests over the weekend.
Authorities also have revoked the accreditation of many Belarusian journalists and deported some foreign journalists, including two Moscow-based Associated Press journalists.
Kolesnikova entered the spotlight in July, when her boss, Viktor Barbariko - initially considered the leading opposition challenger to Lukashenko - was arrested and prevented from running against Lukashenko.More news: What to expect from Apple iPhone 12 in September Event 2020