EVEN by standards of recent palace intrigues, it was a dizzying Saturday in Saudi Arabia. On November 4th the kingdom announced that scores of people had been held in a massive anti-corruption sweep. The best-known figure was Prince Alwaleed bin Talal, a billionaire businessman and investor who owns big chunks of American firms such as Citigroup and News Corporation. But the detainees also include ten other princes; dozens of current and former ministers; the chairman of the Saudi Binladin Group, a construction conglomerate; and the owner of MBC, the largest satellite network in the Middle East. Some are reportedly being held in Riyadh’s opulent Ritz-Carlton, which told paying guests to leave and stopped accepting new bookings. Private jets were grounded in a bid to stop wealthy businessmen from leaving the kingdom.

Even more significant is the separate move to sack several ministers, notably Prince Mutaib bin Abdullah, the commander of the National Guard, consolidating the power of the young crown prince, Muhammad bin Salman (often known as MBS), the hugely ambitious son of King Salman.

To call all of this unprecedented would be an understatement. For decades, Saudi kings tried to forge consensus within the sprawling royal family. Change was incremental and power was divided, particularly among members of the so-called Sudairi Seven branch—the sons of King Abdulaziz, the founder of the state, and his favourite wife, Hussa bint Ahmed Al Sudairi.

Power among the brothers has long been balanced delicately. One Sudairi, Prince Sultan, served as defence minister for 48 years. Another, Prince Nayef, and later his son Muhammad, controlled the interior ministry for more than four decades. And since 1963 the National Guard was the preserve of Prince (later King) Abdullah and his clan.

All three positions are now under the control of MBS. The crown prince became defence minister just hours after his father (also a Sudairi) ascended the throne in 2015. In June the ruling duo sacked the interior minister, Muhammad bin Nayef, the former crown prince, and placed him under house arrest. On Saturday they finished the job by sidelining Prince Mutaib, the second son of the late King Abdullah, who had once been mooted as a possible future king.

The shake-up of the top economic and military posts officially bore the name of the fading monarch. But real power rests with his son. Just 32, he has gathered unprecedented power and is attempting to ram through an ambitious social and economic agenda to modernise the country and wean it off oil.

In September police rounded up dozens of critics, from Muslim clerics to human-rights activists. Then the king abruptly decreed that women would be allowed to drive next year, ending a decades-long ban. A few weeks later MBS wooed investors at a glitzy conference in Riyadh, where he spoke of the need for “moderate Islam” and announced plans for a $500bn economic zone, called Neom, which would be staffed largely by robots. He has a long list of other economic and cultural reforms, from opening cinemas to offering part of Aramco, the state-owned oil giant, in an IPO.

The domestic turmoil comes at an already fraught moment for Saudi Arabia. A ruinous two-and-a-half-year war in Yemen, meant to crush Houthi rebels who ousted the government in 2015, has plunged that country into a humanitarian crisis. Though the Houthis have lost territory, they still control Sana’a, the capital, and most of northern Yemen. As the Saudi shake-up began on Saturday night, they fired a ballistic missile at Riyadh. (It was intercepted.) Just as unsuccessful is the five-month-old blockade of Qatar, which was meant to force the gas-rich emirate to drop its support for Islamist groups.

Meanwhile Saudi Arabia’s arch-rival, Iran, is gaining influence across the region. Indeed, Saturday’s wild news started with the resignation of Saad Hariri, Lebanon’s pro-Saudi prime minister. Mr Hariri announced the move from Riyadh, blaming Iran’s malign influence on his country and saying he feared being assassinated, as his father, a previous prime minister, was in 2005.

Many ask themselves whether the Saudi palace purges are a sign of strength, or reflect nervousness about internal resistance to MBS’s wrenching changes. Few believe the official talk of a clamp-down on corruption. In many ways, the latest arrests seem unnecessary. Aside from Prince Mutaib, the blacklisted officials are a weak lot. Though Prince Waleed is rich, brash and outspoken, he is also an outsider, a prince with little influence on Saudi policy.

Still, the arrests send a message to other would-be critics. MBS, more than any other leader in decades, has fashioned himself the sole face of Saudi policy. If his ambitious plans falter, Saudis will know whom to blame.

Correction (November 6th 2017): Owing to an editing error, a preliminary version of this story was published on November 5th.