The mayor of Zagreb and several of his colleagues have been arrested on corruption and abuse of office charges.
Milan Bandic, a former Social Democrat, has been at the helm of the Croatian capital since 2000.
He has been linked to a number of anti-graft probes in the past but has never before been detained.
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.