(CNN) Tens of thousands of Czechs filled Prague's Letna Plain park on Sunday, taking part in what its organizers described as "the biggest protest since the fall of Communism."

They were demanding the resignation of Prime Minister Andrej Babis , a controversial politician accused of fraud and collaborating with the Communist-era secret police, and his justice secretary Marie Benesova.

The billionaire tycoon has been dubbed the "Czech Trump" by some Czech media outlets because of his vast business empire and populist leanings.

A crowd that was estimated by organizers to be around 250,000 people took over the plain overlooking Prague's historical center, the site of the mass protests that ultimately led to the fall of the country's Communist regime three decades ago.

A no confidence vote called for by opposition parties has been scheduled for Wednesday.

A series of speakers denounced Babis and called for his departure. Several speakers also criticized the country's President Milos Zeman , and urged those in attendance to take steps to protect democracy.

Babis' role in the European Union's failure to agree a net zero carbon emission target was also mentioned by one of the speakers. A plan to set the goal for 2050 was vetoed by four countries, including the Czech Republic, at a summit on Thursday.

Babis told Czech media on Sunday that while he respected people's right to have an opinion, he didn't understand the reasons behind the protests.

The demonstration was part of a series of anti-Babis protests that have been going on for weeks.

Babis has been accused of fraud related to EU subsidies received by his former agricultural business empire. An investigation ended in the police proposing criminal charges against him. Prosecutors are now deciding whether to indict the Prime Minister. He has denied the accusations.

Separately, the European Commission is investigating whether Babis had a conflict of interest because of his role as the PM and the owner of a major business receiving EU funding.

A draft report by the commission, leaked to Czech media earlier this month, said Babis' business should not have access to EU funding, and suggested that the money it had received in the past might need to be returned by the Czech Republic.

The Commission said it would not comment on ongoing audits. The Czech Finance Ministry confirmed it had received a preliminary audit report, but would not comment further.

Protesters in Prague are demanding the resignation of Prime Minister Andrej Babis.

Babis rejected the allegations and said the audit was "an attack on the Czech Republic." Apart from the resignations of Babis and Benesova, the protesters are also demanding that Babis' business empire Agrofert stop receiving subsidies, tax breaks, investment incentives and public tenders.

In addition to being Prime Minister and an agricultural tycoon, Babis is also the owner of Mafra, a major Czech publishing house that owns two of the nation's biggest daily newspapers. The protesters want to see him sell his stake in the media.

They are also asking lawmakers to push for new safeguards for the country's justice system.

Protesters at Letna Plain on November 25, 1989. Three decades later, the plain was again filled with demonstrators.

Who is Babis?

Babis is a divisive figure in the Czech Republic. The billionaire tycoon first stormed into Czech politics in 2012 and became Prime Minister in 2017. His populist ANO movement then received nearly 30% of the vote -- almost three times more than the conservative party ODS that came in second.

Another Babis controversy relates to his alleged role as a secret agent for the Communist-era secret police, the StB. The Slovak-born politician was identified as an agent operating under the codename "Bures" by historians using archival documents.

Babis has rejected the claims and sued for defamation, but in 2018, a court in Slovakia dismissed his complaints, ruling that he was not wrongly identified as an agent in the archival documents.

"We think it unacceptable that ... 29 years after the Velvet Revolution, an StB agent would be the Prime Minister of our country. We will not pretend that that is normal, we demand his resignation," the organizers of the protests said.

Despite the controversies, Babis continues to be relatively popular among Czech voters. ANO was the clear winner in the recent European Parliament elections, securing 21% of the vote.

The organizers have already called for more protests.