A general view of the Slovak Parliament in Bratislava, Slovakia, January 26, 2016. REUTERS/David W Cerny

Dec 12 (Reuters) - Slovakia's parliament passed legislation on whistleblowers and crown witnesses that the opposition warned threaten the rule of law, and which led to scuffles and angry exchanges between politicians late on Thursday and Friday.

The European Commission also raised concerns about the changes, put forward by pro-Russian Prime Minister Robert Fico's government say his changes and backed by leftist-nationalist lawmakers.

Video footage posted by a lawmaker showed opposition politicians whistling and jeering and heated one-on-one arguments in a Thursday evening vote over criminal codes, including the status of crown witnesses.

One government deputy threw a plastic bottle at an opponent, while news website Dennik N said journalists heard another deputy screaming he was being choked in the melee.

On Friday, lawmakers returned to their benches and ruling parties approved a revamp of the country's whistleblower office, even though President Peter Pellegrini had vetoed the law on Thursday, which he said risked halting EU subsidies and did not need to be rushed.

EU CONCERNS

Fico's government, in power since 2023, had previously weakened criminal codes for financial crime, revamped the public broadcaster and pushed constitutional changes asserting national sovereignty over some EU laws.

The government argues the current whistleblower agency UOO had been politically abused in the past - the same reasoning Fico has used for previous legislative changes.

The EU's executive, European Commission, said on Friday it regretted the parliament did not allow for thorough consultation after the Commission previous voiced "strong concerns" about the law.

"This regards notably the dissolution of the existing Whistleblower Protection Office and the resulting early termination of the mandate of the head of office," it said in an emailed response. "The Commission will now analyse the law and decide on the appropriate next steps."

The ruling parties have said concerns had been addressed by modifications, although the change of leadership remained.

OPPOSITION CALLS SESSION 'MAFIA NIGHT'

Slovakia has become more politically charged since Fico - a four-time prime minister - returned to power in the country of 5.4 million. He survived being shot in May 2024 by a man upset over his policies, including a pullback of military aid to Ukraine.

Opposition says the whistleblower bill is "revenge" after the UOO fined the Interior Ministry in cases involving police officers who were reassigned during corruption investigations without the office's consent.

Separate criminal code changes fast-tracked on Thursday evening tightened rules for "crown witness" testimony. Critics say this will help a senior Fico ally who is under investigation.

Michal Simecka, leader of the biggest opposition party Progressive Slovakia, called the parliament session "mafia night".

"We are following a massacre of the rule of law in Slovakia," he said in a statement.

(Reporting by Jason Hovet and Jan Lopatka in Prague; Editing by Alexandra Hudson and Alex Richardson)