Kolesnikova was one of the campaign partners of the opposition candidate Svetlana Tikhanovskaya, who claimed victory against the long-ruling president, Alexander Lukashenko, in disputed elections on 9 August.

Kolesnikova was reportedly seized soon after 10am local time while walking close to Minsk’s national art museum. Three other members of the opposition coordination council have also vanished, in what appears to be a targeted attempt by the authorities to wipe out the protest movement.

Kolesnikova is the most prominent political figure still inside Belarus.

Lukashenko’s victory – in a poll widely seen as rigged – has sparked mass protests. On Sunday, more than 100,000 people marched on the president’s residence, calling on him to quit. Riot police wearing balaclavas arrested 633 people. Gangs of pro-government thugs beat up protesters on their way home.

It is unclear who abducted Koselnikova. Her coordination council colleagues who have disappeared include Anton Rodnenkov, Ivan Kravtsov and Maxim Bogretsov. Her press team is also missing.

Speaking to the local news website Tut.by, a woman identified as Anastasia said she spotted Kolesnikova in the street. She said she was about to go up to her and to thank her for her work when she changed her mind, thinking Kolesnikova looked exhausted.

She said: “Then I noticed a dark minivan with the slogan “Svyaz” on the side parked up not far from the museum. I carried on and then heard the sound of a telephone falling on the tarmac. I turned round and saw people in civilian clothes and masks dragging Maria into the van. The phone flew out of her hand. One of them picked it up, jumped into the van and they drove off.”

Her telephone did not answer, Tut.by reported.

Kolesnikova’s press aide, Rodnenkov, confirmed her abduction but vanished himself around 40 minutes later, it reported. Kolesnikova’s allies said they were checking the report of her detention. Police in Minsk were cited by Russia’s Interfax news agency as saying they had not detained her.

Before the election, Kolesnikova had joined forces with the opposition presidential candidate Tikhanovskaya who later fled to Lithuania, and with Veronika Tsepkalo, who has also since left the country. Another leading activist, Olga Kovalkova, arrived in Poland on Saturday, saying she had been told she would face arrest if she stayed in Belarus.

Friends of Kolesnikova’s believe Belarus’s security forces are behind her abduction. They suspect her fate will be similar to that of Kovalkova, who was taken from a remand centre in Minsk by interior ministry representatives and driven into exile. Other members of the Coordination Council may also be thrown out, they suspect.

There was scepticism, however, that Lukashenko’s mafia-style tactics would halt the protests. “They are spontaneous. It’s really a grassroots thing happening in Minsk and all over Belarus. People unite in their local communities. They start their own Telegram chats and discuss where to march next,” one Minsk journalist said.

The person added: “I believe Maria’s kidnapping won’t stop the protests. It will intensify them actually.”

Senior Belarus opposition figures have accused the EU of failing to respond to Lukashenko’s brutal crackdown on peaceful protesters. Andrei Sannikov, who stood against Lukashanko in the 2010 presidential election, and was subsequently jailed, said sanctions were urgently needed.

Earlier on Monday, central bank figures showed Belarus had burned through nearly a sixth of its gold and foreign exchange reserves, or $1.4bn (£1.06bn), in August, as it fought to prop up its rouble currency during the wave of unrest.

Kolesnikova had announced on 31 August that she was forming a new political party, Together, with the team of jailed opposition figure Viktor Babariko with whom she had previously worked.

Kolesnikova, a trained flautist and music teacher, got into politics through running the campaign of another opposition politician, the former banker Viktor Babaryko, who attempted to stand for president against Lukashenko but was jailed and barred from running.

When Tikhanovskaya, an English teacher and translator with no political experience, was unexpectedly allowed to run for president, Kolesnikova and Tsepkalo backed her and spoke alongside her at rallies.

The women came up with signature gestures: for Tikhanovskaya a raised fist, for Kolesnikova a heart formed with her fingers, and for Tsepkalo a victory sign.

Kolesnikova and other members of Babaryko’s campaign team last month announced the creation of a new opposition party called Together.