Masked men detained one of the last high-profile opposition figures still free in Belarus on Wednesday, his colleagues said, the latest in a series of detentions and expulsions of those leading mass protests.

Maxim Znak, who had worked as a lawyer for jailed presidential hopeful Viktor Babaryko, had been due to participate in a video call but did not show up, instead sending the word "masks" to the group, Babaryko's press service said.

It said a witness had also seen Znak, 39, being led down the street near his offices by several men in civilian clothes and wearing masks.

Along with Svetlana Alexievich, a 72-year-old Nobel Prize-winning author, Znak was the last of the seven members of the opposition Coordination Council's governing presidium to remain free.

Others have been detained or forced to leave Belarus, in an intensifying crackdown by President Alexander Lukashenko's regime over a disputed election.

Znak's apparent detention came a day after the most prominent opposition figure still in Belarus, Maria Kolesnikova, was detained at the Ukrainian border after she prevented authorities from expelling her by tearing up her passport and jumping out of a car.

The Coordination Council was set up to ensure a peaceful transfer of power after main opposition candidate Svetlana Tikhanovskaya rejected Lukashenko's claim to have been re-elected to a sixth term in an August 9 vote.

The election has sparked the biggest anti-government demonstrations of Lukashenko's 26-year rule, with tens of thousands taking to the streets for weeks to demand he resign.

Lukashenko's security services have hit back with waves of arrests, deadly violence against protesters and a campaign of intimidation and expulsion against opposition leaders.

Kolesnikova's fate was unknown after the dramatic events on Wednesday, when she and two other members of the Council were taken in a car to the buffer zone between the Belarusian and Ukrainian borders.

- Appeal to Russians -

The other two council members, press secretary Anton Rodnenkov and executive secretary Ivan Kravtsov, told a press conference in Kiev that Kolesnikova had torn her passport "into small pieces" and climbed out of the back window of the car to avoid expulsion.

Border police said she was being held pending an investigation.

Kolesnikova had gone missing on Monday, with witnesses saying she was bundled into a minibus by unidentified masked men on the street in Minsk.

Kolesnikova, 38, is the only one of the trio of women who fronted Tikhanovskaya's campaign still in Belarus.

Tikhanovskaya left the country under pressure from the authorities and was granted refuge in EU member state Lithuania. Her other campaign partner, Veronika Tsepkalo, is now in Ukraine.

Tikhanovskaya on Wednesday called on Russians not to believe propaganda trying to "poison" ties between the two peoples and thanked those backing Belarusians' "fight for freedom".

Her first major address to Russians came after Lukashenko gave a wide-ranging interview to a group of journalists from Russian state media on Tuesday.

"It is very important not to damage ties between the two countries," 37-year-old Tikhanovskaya said in the video address.

"Let's not allow propaganda to poison ties between two friendly peoples, and unscrupulous politicians to damage the interests of both Belarus and Russia," she said.

No date has been set, but Lukashenko is preparing to travel to Moscow for talks with Russian President Vladimir Putin.

Putin quickly congratulated Lukashenko on his re-election last month and has offered Russia's support.

In the interview with Russian journalists, Lukashenko said he did not recognise the Council as the opposition and warned that if his government falls, "Russia will be next".