A new anti-corruption body, headed by Crown Prince Mohammed bin Salman, detained 11 princes, four sitting ministers and dozens of ex-ministers.
Prince Alwaleed bin Talal, a billionaire with investments in Twitter and Apple, is among those held.
Separately King Salman replaced the national guard and the navy chiefs.
The new anti-corruption committee has the power to issue arrest warrants and travel bans.
Saudi broadcaster Al-Arabiya said fresh investigations had been launched into the 2009 Jeddah floods and the outbreak of the Mers virus which emerged in Saudi Arabia in 2012 – but analysts see the detentions as a clear move by the crown prince to strengthen his power base.
