Kolesnikova's representatives said they could not reach her after unconfirmed reports emerged that she was kidnapped in Minsk.
Belarusian opposition leader Maria Kolesnikova has reportedly been detained in Belarus after refusing to leave the country.
She and two other opposition staffers disappeared Monday morning and bystanders said they were kidnapped in the centre of Minsk.
Belarus' border committee later said Kolesnikova was detained at the border early on Tuesday.
The two staffers who also disappeared on Monday, Anton Rodnenkov and Ivan Kravtsov, later turned up in Kyiv. The Ukrainian government confirmed they passed a border check but said Kolesnikova did not.
Ukraine's deputy interior minister Anton Gerashchenko said that this was a "forced expulsion" from Belarus. He said Kolesnikova did not have the right documents for her to leave Belarus.
Opposition activists said Kolesnikova ripped up her passport to avoid being forced to leave, according to AP.
Kolesnikova is a leader of the Coordination Council which was set up by the opposition in Belarus to create dialogue with authoritarian leader Alexander Lukashenko over a transition of power following last month's disputed election.
The 38-year-old ran Viktor Babariko's opposition campaign but he was detained prior to the presidential elections. She then joined forces with candidate Sviatlana Tsikhanouskaya.
Tens of thousands of Belarusians protested against Lukashenko once again on Sunday marking the fifth week of protests against the country's leader.
Tsikhanouskaya, who was the main opposition candidate in the August election, has urged the international community to impose sanctions on “the individuals that committed electoral violations and crimes against humanity” following a violent crackdown on protests.
European states demand answers over disappearance
Germany and Britain expressed concerns over the disappearance of the Belarusian opposition figure, after she went missing on Monday and before the events at the Ukrainian border.
"We are very worried (about) Ms Kolesnikova," said German Foreign Minister Heiko Maas as quoted by the tabloid Bild. "We demand to know where she is and we call for the release of all political prisoners in Belarus."
He added that Germany - which currently holds the presidency of the European Union - is working "relentlessly" on a series of sanctions against Alexander Lukashenko's regime.
Meanwhile, British Foreign Secretary Dominic Raab tweeted that the government must make the return of Kolesnikova its highest priority.
Activists expressed further concern on Monday evening when Kolesnikova had still not reappeared. "More than eight hours have passed and we still don't know where Maria is and what's happened to her," said Maxime Znak, a member of the opposition Coordination Council, said in a video posted on YouTube.
The Belarusian interior ministry told AFP on Monday it had no information on an arrest.
Gleb German, opposition candidate Viktor Babariko's press representative, confirmed to Euronews earlier in the day that Kolesnikova, who runs Babariko's headquarters, had not been able to be reached since 10 a.m. Monday.
"The most important thing now is to establish a connection with Maria, Ivan and Anton in order to understand that they are okay and why they have been detained," German said in a statement to Euronews. German said some Belarusian agencies denied having detained them.
What happened Monday morning?
A bystander near the National Art Museum told independent Belarusian media site tut.by that she saw Kolesnikova taken by masked people in civilian clothes. The bystander told tut.by that Kolesnikova was put in a minibus.
Lithuanian foreign minister Linas Linkevicius said the reports showed "the outgoing leadership is trying cynically to eliminate one by one."
"The kidnapping of M.Kalesnikava in downtown of #Minsk is a disgrace. Stalinist NKVD methods are being applied in 21st century’s Europe. She must be released immediately," Linkevicius tweeted.
Watch Euronews' previous interview with Kolesnikova in the video player below: