(CNN) Tens of thousands of Czechs took to the streets of Prague on Tuesday, 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 demonstration was part of a series of anti-Babis protests that have been going on for weeks.

A crowd that was estimated by organizers to be around 120,000 people filled Wenceslas Square in the Czech capital, the site of the protests that ultimately led to the fall of the country's Communist regime three decades ago.

What are the protests about?

Babis, mostly. The Prime Minister is at the center of a number of controversies and the protesters want him gone.

Organizers said the protest was the country's biggest since the fall of Communism in 1989.

He 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. Babis has denied the accusations.

Separately, the European Commission is investigating whether Babis was in 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 last week, 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.

Speaking in parliament on Tuesday, Babis rejected the allegations and said the audit was "an attack on the Czech Republic."

What do the protesters want?

Apart from the resignations of Babis and Benesova, the organizers are demanding that Babis' business empire Agrofert stops 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.

The rally in Prague demanded the resignation of Czech Prime Minister Andrej Babis.

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

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 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.

What happens next?

The organizers have already called for more protests. They are urging people in cities other than Prague to march next week, and are planning another mass protest in the capital on June 23.

Babis has been accused of fraud related to EU subsidies.

This time, they are planning to move to Letna Plain, a huge open space atop a hill overlooking the historical center of the city, which is capable of holding many more people than Wenceslas Square.

The leaders of Czech opposition parties said Wednesday they would meet to decide whether to press for a confidence vote.