Prague, Nov 18 (AFP) The Czech Republic's billionaire prime minister expressed regret over his communist past on Sunday while Slovakia's liberal president hailed the sacrifices communist-era dissidents made for democracy as their nations celebrated 30 years since the Velvet Revolution toppled communism in then-Czechoslovakia.

Populist Czech premier Andrej Babis, a Communist Party member in the 1980s, and Slovak President Zuzana Caputova, a former environmental activist, both paid tribute to the peaceful uprising that ushered in democratic reforms to the former Soviet satellite in 1989.

But three decades after staging the mass protests that freed them from communism, Czechs and Slovaks have hit the streets again over the past year, concerned that pervasive corruption and politicians with communist roots are corroding democracy.

An estimated quarter-million Czechs flooded central Prague on Saturday to mark the anniversary and demand that Babis resign over allegations of graft and that he was once a communist secret agent. He has strongly denied the accusations.

"As you surely know, I was a Communist Party member. I'm not proud of that," Babis said at a Sunday ceremony in Prague attended by the prime ministers of Hungary, Poland and Slovakia and by German parliament speaker Wolfgang Schaeuble.

He said he "wasn't as brave" as Vaclav Havel, the dissident playwright elected the president of Czechoslovakia in 1989, and thanked those behind the Velvet Revolution protests.

"I'm standing here today as the prime minister elected in a free, democratic election, and therefore I want to, at least now, express my gratitude and humility," said Babis, whose minority government now relies on the tacit support of Communist Party lawmakers to survive in parliament.

Speaking at Bratislava's Gate of Freedom memorial dedicated to some 400 Slovaks shot by the regime they tried to flee to neighbouring Austria, President Caputova hailed communist-era "prisoners,(who were) persecuted and punished; fighting for your own freedom, you won ours."

The 46-year-old anti-corruption and environmental activist was elected as Slovakia's first female president in March on the back of mass street protests over the 2018 murder of investigative journalist Jan Kuciak, gunned down as he probed high-level graft.

"Democracy is an opportunity, not a guarantee of success," Caputova warned on the eve of the anniversary.

Czechs laid a carpet of flickering candles at the foot of the Velvet Revolution memorial in central Prague on Sunday.

Svatopluk Cerny, a 53-year-old IT specialist, told AFP that although 1989 brought "an absolute sea change" regarding freedoms, under democracy he sees "many squandered chances... whether in politics or the environment." Several thousand people jingled keys and shone lights on their mobile phones in central Bratislava Sunday demanding justice for Kuciak.

Jan Budaj, a former dissident leader turned conservative opposition lawmaker, called for more action to safeguard democracy.

"Freedom has already been achieved, but now it's a matter of fighting for justice, and a functional rule of law," he said, calling for Kuciak's killers to be brought to justice.

Slovak businessman Marian Kocner has been charged with ordering the hit on Kuciak, who was investigating the property developer's murky business dealings and ties to top politicians.

Politicians with roots in communism have never quite disappeared and are now joining forces with populists in both Prague and Bratislava.

Babis took office after his populist ANO (YES) party won the 2017 general election, promising generous public spending and anti-corruption measures in the country of 10.6 million.

The fifth wealthiest Czech according to Forbes, Babis faces a string of graft allegations and a conflict-of-interest probe by the European Commission centred on Agrofert, his sprawling farming, media and chemicals holding.

He is also tagged as an agent in secret police files from the 1980s but has flatly denied any knowing cooperation.

ANO still tops opinion polls with around 30 percent support, despite the controversy.

Over the last year, Slovakia has also witnessed its largest protests since the Velvet Revolution, triggered by Kuciak's gangland-style killing as he probed alleged ties between senior politicians and the Italian mafia.

With political roots in the Communist Party, Robert Fico, leader of Slovakia's governing Smer-SD populist social democrats, is widely seen as calling the political shots despite not being in government.

He was forced to quit as prime minister last year in the wake of Kuciak's murder but his Smer-SD is still on course to win a general election due in February.

The Velvet Revolution saw unprecedented protests and a general strike end four decades of Soviet-imposed totalitarianism in the former Czechoslovakia, just weeks after the Berlin Wall crumbled.

On November 17, 1989, Communist police brutally crushed a students' march, sparking a student strike and the creation of an opposition movement which then negotiated the Communist Party's departure from politics.

In late December 1989, Havel, then the opposition leader, was elected president of Czechoslovakia, which then peacefully split into the Czech Republic and Slovakia in 1993.

The neighbours joined NATO and the EU, with Bratislava also becoming a eurozone member in 2009. (AFP)

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