The protesters demanded the resignation of Interior Minister Robert Kalinak, who is a close ally of Fico's, police chief Tibor Gaspar and other officials they allege have prevented proper investigations of corruption scandals.
President Andrej Kiska was supporting the student- organized demonstration.
Today's rally is the biggest protest in Slovakia since 2012, when a file allegedly compiled by the country's SIS spy agency appeared online. The file, known as "Gorilla," suggested that a financial group had bribed government and opposition politicians to win lucrative privatization deals.