The protests, called by pro-western liberal and conservative opposition parties and civic activists, were the fourth to take place since Fico proposed rapidly reforming prosecution and criminal law in December, something that has also raised concerns at the European Commission and in the United States.

His plan includes scrapping the special prosecutor's office (USP) which investigates organised crime and graft cases, limiting protection for whistleblowers and reducing sentences for financial crimes.

"There is a threat that...justice for all will change into amnesty and protection of people close to the ruling coalition, for deeds past and future," Marian Caucik, deputy leader of the opposition Christian Democrats, said at a rally in Bratislava.

People carrying European Union and Slovak flags gathered in freezing temperatures on a downtown square chanting "Shame!" and "Enough of Fico!". One banner read "Opposition, unite against the Mafia".

Police estimated turnout at 20,000, news website dennikn.sk reported. Rallies in other cities drew hundreds to thousands, Slovak media reported.

Forced to resign after mass protests in 2018 following the murder of anti-corruption journalist Jan Kuciak, Fico returned to power after an election in September.

He has long accused the USP, which was created in 2004, and its leader Daniel Lipsic, a former justice minister in a non-Fico government, of being politically biased against his leftist SMER-SSD party.

Opposition parties prevented Fico's party and its allies from pushing through the proposed reforms before Christmas but parliament has been debating the proposal again this week.

Following a 2020 election win by parties promising to weed out graft, the USP opened a number of cases against business leaders, judicial and police officers.

According to Slovak media, 40 people have been sentenced while another 130 are being investigated or tried.

Apart from the murder of Kuciak, the office has handled a case against former SMER-SSD finance minister and current central bank Governor Peter Kazimir, who has been charged with bribery, which he denies.

It has also overseen abuse of power and other charges against former police chief and now Fico ally and SMER member of parliament Tibor Gaspar. He denies any wrongdoing.

While in opposition, Fico faced police charges, later dropped, that he used information from police and tax authorities to discredit political rivals. He said at the time it was political revenge.

(Reporting by Radovan Stoklasa in Bratislava and Jan Lopatka in Prague; Editing by Kirsten Donovan)

By Radovan Stoklasa and Jan Lopatka