KYIV, UKRAINE -- A leading opposition activist and several other members of an opposition council in Belarus went missing Monday and their colleagues feared they were detained as part of the authorities' efforts to squelch nearly a month of protests against the re-election of the country's authoritarian leader.

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 that drew an estimated 100,000 protesters pushing for the resignation of Lukashenko, who extended his 26-year rule in the Aug. 9 election that the opposition sees as rigged.

A council member, Maxim Znak, said Kolesnikova's colleagues fear she was detained. Two other members of the council also couldn't be reached on Monday, according to Znak.

Lithuanian Foreign Minister Linas Linkevicius tweeted that Kolesnikova's disappearance is part of the Belarusian authorities' effort to "cynically eliminate one by one" and called for her immediate release.

The Belarusian Interior Ministry said it hasn't detained Kolesnikova, while other law enforcement agencies have kept mum.

After a brutal crackdown in the first few days after the vote that stoked international outrage and swelled the ranks of protesters, the authorities have switched to threats and selective arrests of opposition activists and demonstrators.

Sunday's demonstrations in Minsk and other cities went on despite the authorities' warning that participants could face reprisals. The Interior Ministry said 633 people were detained Sunday for taking part in unsanctioned protests.

Targeting the protest leaders, Belarusian prosecutors have opened a criminal probe of the Coordination Council that opposition activists set up after the election to try to negotiate a transition of power. Two of its members were given 10-day jail sentences on charges of staging unsanctioned protests last week. One of them, Olga Kovalkova, said she was pressured to leave for Poland over the weekend after being threatened with a longer jail term.

Sviatlana Tsikhanouskaya, the main opposition challenger in the election, who moved to Lithuania a day after the vote under pressure from authorities, said Monday that Kolesnikova's disappearance marked another attempt by the government to intimidate the opposition.

"It's an attempt to derail the work of the Coordination Council, but we will not be stopped," she said. "The more they try to scare us, the more people will take to the streets. We will continue our struggle and push for the release of all political prisoners and a new, fair election."

Last week, Kolesnikova announced the creation of a new party, Together. She said the move will help overcome the current crisis, but the party founders' call for constitutional changes has stunned some other opposition council members, who argued that it could divert attention from the main goal of pushing for Lukashenko to step down.

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Vladimir Isachenkov in Moscow and Jan M. Olsen in Copenhagen, Denmark, contributed to this report.