Kalesnikava was detained trying to cross the border while the other two had entered Ukraine.
Three senior opposition organizers who went missing and were feared to have been abducted by security forces to stifle ongoing mass protests against President Alyaksandr Lukashenka turned up at the border with neighboring Ukraine early on September 8.
Opposition Coordination Council presidium member Maryya Kalesnikava, press secretary Anton Randyonkau, and executive secretary Ivan Krautsou arrived at the Alyaksandrauka border checkpoint in the Homel region in the same car at around 5:00 a.m, RFE/RL wrote.
Belarusian State Border Committee representative Anton Bychkouski initially said all three had left the country.
But Belarusian state television later quoted Bychkouski saying that Kalesnikava was detained trying to cross the border while the other two had entered Ukraine.
All three are key figures on the Coordination Council that has pressed for a peaceful transition of power since election officials declared Lukashenka the runaway winner of an August 9 vote they say was fraudulent.
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Thousands of people have been arrested, journalists have been harassed and expelled, and clips have emerged of Lukashenka's security services brutally abusing detainees.
Members of the Coordination Council and its decision-making presidium have been summoned by police and in some cases sentenced to jail.
An eyewitness reported seeing Kalesnikava swept up by unidentified men from the street and into a minivan in downtown Minsk on September 7.
Acquaintances said contact was lost soon afterward with Randyonkau and Krautsou.
The opposition's leading hope in last month's election was political novice Svyatlana Tsikhanouskaya, who fled into exile in Lithuania days after the vote.
Tsikhanouskaya is scheduled to address the Parliamentary Assembly of the Council of Europe (PACE)'s Committee on Political Affairs and Democracy during an exchange of views on the situation in Belarus later on September 8.
She is also scheduled to visit Warsaw this week to hold meetings with top Polish officials.
Lukashenka, who has served five terms already, has refused to hold talks with his opponents and dismissed calls to hold a new election.
The EU "expects the Belarusian authorities to ensure the immediate release of all detained on political grounds before and after the falsified August 9 presidential election," the bloc's foreign policy chief Josep Borrell said on September 7.
Belarusian authorities acknowledged detaining some 633 protesters as tens of thousands marched in the capital and other cities on September 6 to pressure Lukashenka to leave.