Three of the Belarusian opposition’s top organizers were reported missing Monday, the day after tens of thousands joined protests in the country’s capital that kept up pressure on President Alexander Lukashenko to hold new elections.

“Peaceful actions all across Belarus since Aug. 9 and representing the view of the majority of people are so massive that authorities have begun actively to use methods of terror in order to suppress them,” the united opposition’s coordinating council said on its website Monday.

Maria Kalesnikava, one of the few leading opposition figures still in Belarus, the council’s spokesman Anton Radniankou and its executive secretary Ivan Krautsou have been “kidnapped” by unknown people, according to the statement. Amid a renewed police crackdown on protesters, detentions and the forced expulsion from Belarus of another council member, Volha Kavalkova, the disappearances may deepen the crisis in the country, the council warned.

Why the Strongman of Belarus Is Fighting for Survival: QuickTake

The Belarusian Interior Ministry told the RIA Novosti news service it had no information about detentions of Kalesnikava and the other two opposition members.

The police said they detained 633 people during protests across the country Sunday, and 363 remain in custody pending court hearings. Opponents of the president have taken to the streets daily since the Aug. 9 election in which he claimed a landslide victory.

With protests and calls for strikes slowing Belarus’s economy, the nation’s central bank burned through
$1.4 billion of its foreign currency reserves to prop up the country’s ruble in August, the biggest monthly decline in reserves since 2009.

Several key figures of the opposition were jailed before the election, including ex-banker Viktor Babariko and political blogger Siarhei Tsikhanouski, whose wife Sviatlana Tsikhanouskaya became the leading challenger to Lukashenko and the face of the protests. She is now in exile in Lithuania.

(Updates with opposition council’s statement from second paragraph)