Croatian anti-graft police arrested the mayor of Zagreb, Milan Bandic, and several of his associates on Sunday on suspicion of corruption and abuse of office, Croatian media reported.
The police and the state prosecutor said only that several people from the Zagreb administration had been arrested but did not name them, in line with usual practice.
But all local media and the national televisions named Bandic as one of those arrested. Reports have linked Bandic to a number of corruption probes in the past decade but he has never been interrogated or detained until now.
"After several months of a complex criminal investigation, several persons have been arrested on suspicion of illegal activities in the city of Zagreb and the Zagrebacki Holding (the city's company in charge of communal services)," the state prosecutor's office DORH said in a statement on its website.
It gave no details of possible charges, saying only that "a number of corruption crimes are being investigated".
Bandic, a former Social Democrat who left the party in 2009 to run as an independent candidate in the presidential election that year, has been at the helm of the Croatian capital since 2000.
Zagreb has almost a quarter of the country's 4.4 million people and is the financial and economic capital of the former Yugoslav republic, which joined the EU in July last year.
Croatia launched an extensive anti-corruption drive in 2010 to boost its faltering EU membership bid. It has since tried and convicted several top state officials, including the former Prime Minister Ivo Sanader.