(CNN) For many Czechs, Sunday's anniversary of the Velvet Revolution will be a bittersweet moment. Three decades after the collapse of the Communist regime, people will mark a dramatic moment in history amid allegations, confirmed by a court decision, that their prime minister collaborated with the StB, the Communist-era secret police.

Police said 250,000 attended anti-government demonstrations in Prague on Saturday, one day before the anniversary, when a non-violent demonstration led by students emboldened the nation to rise up against Communism.

They are demanding the resignation of PM Andrej Babis, a business tycoon who is listed as an StB agent in its official archives. Babis has consistently denied cooperating with the StB knowingly, saying he has been wrongly identified in the documents.

Pensioners Miloslava and Pavel Šimáček took part in the mass protests 30 years ago. They came back on Saturday because they were opposed to Babis being PM.

Carrying home-made banners, they paused to listen to the national anthem playing from the sound system.

"I'm very unhappy about the fact that the Prime Minister is a former StB agent and Communist. He has no self reflection whatsoever," Pavel Šimáček said. His wife Miloslava added: "Back then, 30 years ago, if someone told me this would the case, I wouldn't have believed it. We knew everything won't be ideal, but this is unacceptable."

Miloslava and Pavel Šimáček march in Prague on Saturday -- 30 years after they took part in the Velvet Revolution protests.

The organizers of the protests, a group called "A Million Moments for Democracy" has coordinated some of the biggest demonstrations since 1989 in the past few months. More than 432,000 people have also signed its petition calling for Babis to resign.

"We consider it unacceptable ... for an StB agent to be the Prime Minister," the petition said. "We are not going to pretend that it's normal. We want Andrej Babis to step down!"

Babis, a Slovak-born millionaire, has admitted to meeting with StB agents in the 1980s, when he was a member of the Communist Party working for a foreign trade company -- a cushy job that allowed him to travel abroad, a perk that was unthinkable for most Czechoslovaks at that time.

However, he has denied cooperating with the StB knowingly.

According to official files held by the Slovak Nation's Memory Institute, Babis agreed to become a collaborator during an hour-and-a-half long meeting with StB officials at a wine bar in Bratislava in November 1982.

The historical files describes him as being worried his coworkers would find out about his collaboration, something he thought could hurt his career. But, according to the records, he signed up anyway, adopting the code name "Bures."

Babis has fought a bitter legal battle to have his name removed from the files, a bid that was finally rejected by the Slovak Constitutional Court last year. The institute presented 12 separate files signed by some 40 former StB officials as evidence against Babis.

Vote winner

While many in the Czech Republic disagree with Babis and want him gone, the PM has plenty of supporters too. His political party ANO won the European Parliament elections in May with 20% of the vote.

In the last parliamentary election in 2017, the group secured almost 30% of the vote, way more than the second-biggest party ODS which got just over 11%. His populist politics included a firm rejection of the EU's proposal for a new immigration quota that would split refugees among the member states. He was among the four leaders who, in June, rejected Europe's proposal to slash carbon emissions

The protesters are now urging the fragmented opposition parties to come up with a plan to defeat Babis, and have pledged to keep protesting until their demands are met.

Their criticism goes way beyond Babis' alleged past.

"The justice system and the public media are in danger, and the president, disregarding the constitution , is promising, if ever needed, a presidential pardon to the Prime Minister who has been in a huge conflict of interest," they said in a statement on the website.

Babis has been accused of fraud related to EU subsidies received more than a decade ago by his former agricultural business empire Agrofert.

An investigation ended in the police proposing criminal charges against him. But this September, on the day of Babis' 65th birthday, prosecutors put the case on hold and later decided to drop it all together. Babis has denied the accusations.

ANO has not responded to a request for comment.

The European Union is also investigating Babis for a potential conflict of interest. Babis transferred the ownership of Agrofert into trust funds before taking the office and the EU is investigating whether he is still profiting from the business.

The vast empire includes a large number of food producers, meaning Czech supermarkets are full of products made by companies that trace their ownership to the PM.

App developer Vytrhlík was curious about the scale of Babis' business empire and devised a smartphone app called "Bez Andreje," Czech for "Without Andrej."

It allows people to scan the barcode of any product to check whether it was made by one of the Agrofert companies. It has been downloaded more than 250,000 times, Vytrhlík said.

The organizers of Saturday's protests were hoping for big crowds.

"We don't have many reasons to be optimistic, 30 years after the Velvet Revolution," the organizers, who are mostly students, said in a statement. "How else shall we celebrate the anniversary than by raising our voices in defense of democracy?"