In brief comments to journalists, Necas confirmed that Jana Nagyova, the director of his office, was among those arrested. He said he would stand by her.
"At the moment, I have no information that would prove any criminal activity of my collaborators," Necas said.
Police spokesman Pavel Hantak declined to identify any of those arrested.
Necas called on the police and prosecutors to "immediately give reasons for such a massive action."
The Social Democrats, the main opposition party, called for Necas to resign.
President Milos Zeman said he would discuss the situation with Necas, the nation's police chief, the top state prosecutor, Justice Minister Pavel Blazek and Bohuslav Sobotka, chairman of the Social Democrats, tomorrow.