The reported arrest of Nasser bin Aqeel al-Tayyar followed the detention of dozens of top Saudis including billionaire investor Prince Alwaleed bin Talal in a crackdown that the attorney general described as "phase one".

The purge is the latest in a series of dramatic steps by Crown Prince Mohammed bin Salman to assert Saudi influence internationally and amass more power for himself at home.

The campaign lengthens an already daunting list of challenges undertaken by the 32-year-old since his father, King Salman, ascended the throne in 2015, including going to war in Yemen, cranking up Riyadh's confrontation with arch-foe Iran and reforming the economy to lessen its reliance on oil.

Both allies and adversaries are quietly astounded that a kingdom once obsessed with stability has acquired such a taste for assertive - some would say impulsive - policy-making.