At least 15,000 people demonstrated in Slovakia's capital on Tuesday as protests grew against plans of Prime Minister Robert Fico's government to fast-track criminal law changes that include scrapping a special prosecutor's office focused on corruption, Paralel.Az reports citing Reuters.

Opposition parties led protests in cities around the central European country of 5.4 million for a third straight week, with protesters in Bratislava waving Slovak and European Union flags and chanting "Enough of Fico".

Dennik N news website said 15,000 to 18,000 protested, up from 10,000 a week ago.

The new government, in power since October, is seeking to speed through proposals in parliament that include scrapping a special prosecutor's office for high-profile graft cases, limiting protection of whistleblowers and reducing sentences for financial crimes.

The European Commission has warned it will take action against Slovakia if it violates EU laws.

Opposition parties have promised to do all they can to block the reform plans.

"(The government is) underestimating us," Michal Simecka, head of the biggest opposition party in parliament, Progressive Slovakia, told the crowd.

"We have to show them they are wrong. We have to show them that we are defending justice and democracy, even in the winter, even before Christmas."

Patrik Kamencay, 25, said he joined the protest because he does not like how Slovakia looked in the EU.