Thousands of Slovaks rallied in Bratislava on Monday to protest against corruption and demand the resignation of the interior minister over his ties with a developer under investigation for tax fraud.
Slovakia's economy has boomed and living standards have risen sharply since it joined the European Union in 2004, but many Slovaks say their country fails to defend the rule of law, especially in punishing graft.
The protesters, chanting "Kalinak, resign" and "Leave our state alone", carrying Slovak and EU flags, largely targeted Interior Minister Robert Kalinak.
He has resisted pressure to step down over his business relations with real estate developer Ladislav Basternak, who is being investigated over possible tax fraud.
Kalinak has said he bought a 17 percent stake in one of the companies belonging to the developer. Both he and Basternak have denied any wrongdoing.
The protesters also demanded the resignations of police chief Tibor Gaspar and special prosecutor Dusan Kovacik for not doing enough to root out corruption.
The protest against Kalinak, number two in Fico's Smer party, came as civic protests mushroom across the EU's eastern wing.
Poland, Hungary and Romania have seen large anti-government protests in recent months, and thousands of Czechs took to the streets in May to protest against the finance minister.
A frequent theme is corruption and the inability of justice systems to put powerful suspects on trial.
Slovak media estimated turnout between 5,000 and 10,000, making it one of the largest protests since Fico took power in 2012. There was no official police estimate.
"Public sensitivity to corruption is growing and the prime minister has also reflected it in his activities — he has appointed a new head of an anti-corruption unit, spoken at anti-corruption seminars but a symbolic closure is still missing," political analyst Martin Slosiarik told Reuters.
"Many people don't believe justice is for all, they stopped believing in the principles of democracy, which has helped extremist parties," he said.
The far-right People's Party-Our Slovakia group won its first parliamentary seats last year, in an election which gave Fico a second consecutive term as prime minister but cost his party a parliamentary majority.
Fico said on Monday he supported freedom of speech. He backed Kalinak last year when the minister first faced protests demanding his resignation.
The government has stepped up anti-corruption efforts, introducing a 35 percent tax on dividends from companies in tax havens and banning firms with unclear ownership from doing business with the state.