Belarus on Thursday launched a criminal case against a new opposition body, accusing it of an illegal attempt to seize power.

This came a day after President Alexander Lukashenko threatened to sweep the streets of protesters who rejected his re-election.

Belarus has been facing its biggest political crisis since the breakup of the Soviet Union, with tens of thousands of demonstrators rejecting Lukashenko’s victory in an Aug. 9 vote his opponents say was rigged.

Opponents of Lukashenko, a gruff former collective farm boss in power for 26 years, unveiled the Coordination Council on Tuesday with the stated aim of negotiating a transfer of power.

Its dozens of members include a Nobel Prize-winning author and the ousted head of Minsk’s main drama theatre, as well as an exiled presidential candidate, Sviatlana Tsikhanouskaya, whose followers say she won the election.

“The general prosecutor’s office described the body as designed to seize power, calling the act of setting it up a threat to national security,’’ Russia’s RIA News Agency reported.

No individuals were named as suspects in the case.

The council issued a statement rejecting the accusations and saying its efforts were lawful.

“The accusation is completely baseless and without foundation. Our goal is to resolve the crisis without conflict.

“We are not calling for the seizure of power,” council member Syarhei Dyleuski, leader of a committee of striking workers at the Minsk Tractor Factory, told Reuters.

After days of huge rallies drawing tens of thousands of demonstrators, protests were diminished but not halted.

On Wednesday, Lukashenko announced that he had ordered police to clear the protesters off the streets of the capital, although no action was taken against hundreds of protesters who staged a rally in front of the police headquarters later that day.

By lunchtime, there was still no sign of a decisive security operation.

Bigger rallies are expected again over the weekend.

Tsikhanouskaya, a 37-year-old political novice who emerged as the consensus opposition candidate after better-known figures were barred from standing, including her jailed activist husband, had fled to neighbouring Lithuania.

He assured her that the government, together with its partners in Poland, Latvia and Estonia, were doing and would do everything so that there would be free and fair elections in Belarus.

“Also her children can as soon as possible hug their dad in freedom,” he wrote on his Facebook page.

The crisis in Belarus, Russia’s most loyal neighbour, is a test for the Kremlin, which has to decide whether to stick with Lukashenko or try to manage a transfer of power.

It also posed a challenge to Western leaders trying to avert violence six years after a popular uprising in neighbouring Ukraine drew Russian military intervention and triggered Europe’s deadliest ongoing conflict.

Of all Russia’s former Soviet neighbours, Belarus has the closest political, economic and cultural relationship with Moscow, and its heavily fortified borders with Latvia, Lithuania and Poland are major frontiers of NATO.

The European Union has rejected Lukashenko’s re-election and announced sanctions on Wednesday against some Belarus officials it blames for election fraud and abuse of protesters.

EU members bordering on Belarus have called for a tough line towards Lukashenko, but European officials are wary of strong action that might provoke Moscow into a military response.