© Amr Nabil/Associated Press

Saudi Arabian authorities detained two of the kingdom’s most prominent figures for an alleged coup attempt, further consolidating the power of the king’s son, Crown Prince Mohammed bin Salman, and clearing away once-formidable rivals to the throne, according to people familiar with the matter.

The detentions occurred early Friday morning when guards from the royal court wearing masks and dressed in black arrived at the homes of the two men, took them into custody and searched their homes, according to people familiar with the matter.

Prince Ahmed bin Abdulaziz al Saud, a brother of Saudi King Salman, and Prince Mohammed bin Nayef bin Abdulaziz al Saud, the king’s nephew known as MBN, were both accused of treason, the people said. The guards also arrested one of MBN’s brothers.

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The Saudi royal court accused the two men of plotting a coup to unseat the king and crown prince, according to people familiar with the situation.



The two men who potentially had once been in line for the throne are now under threat of lifetime imprisonment or execution, said people familiar with the situation. The details of the alleged coup attempt couldn’t be learned.

Prince Ahmed and MBN spent time as minister of the interior, a powerful position with oversight of troops and Saudi Arabia’s large intelligence service. MBN had close ties to U.S. intelligence and was respected for his knowledge of security and terror threats in the Middle East. But over the past few years, their standing in the royal family has diminished as King Salman consolidated power and installed his son Mohammed bin Salman—known as MBS—as crown prince and the kingdom’s day-to-day ruler.

The arrests cast aside two men who could have been rivals to MBS’s claim to the throne if 84-year-old King Salman died or decided to abdicate. MBN had been first in line for the throne as Salman’s crown prince until 2017, when MBS was elevated over him.

Early in MBS’s ascent, his social and economic reforms pleased many Western officials who had long called for the kingdom to lift restrictions on women and foster a more diversified economy, rather than remain heavily reliant on oil.

But the prince’s tactics for cracking down on corruption, silencing dissent and marginalizing rivals have since alienated many in the U.S. and Europe, especially in the time since MBS became crown prince. To curb corruption, MBS launched a surprise dragnet in 2017 that saw many of the kingdom’s most powerful businessmen and royals locked up in the Riyadh Ritz-Carlton Hotel and accused of ripping off the government. Many were only released after agreeing to pay tens of millions of dollars to the royal court.

The next year, Saudi dissident Jamal Khashoggi, a columnist for The Washington Post, was murdered and dismembered in Turkey by men working for MBS. The backlash presented the biggest threat yet to the crown prince.

In a further crackdown on dissent, Prince Mohammed has arrested online critics and used armies of Twitter accounts to criticize people who question his governance. Men working for him were also accused by the U.S. Justice Department last year of paying two Twitter employees for private information on dissidents.

The arrest of MBN follows sharp bipartisan criticism of Saudi Arabia in the U.S. Congress, where support for the kingdom already has waned because of Mr. Khashoggi’s killing and widespread civilian casualties in Yemen caused by a Saudi-led coalition using American-supplied weaponry.

MBN was for years a trusted contact for U.S. security and intelligence officials. As the head of Saudi counterterrorism efforts, he shared information with American counterparts about planned attacks and gained allies throughout the State Department and Central Intelligence Agency. Many U.S. officials were dismayed by his ouster in 2017, said several former intelligence and State Department officials, since they felt he was the U.S.’s most reliable ally within the royal family.

MBN has been under house arrest for much of the time since MBS stepped in front of him, though he did appear at private events during an investment conference last fall, said people familiar with the matter.

Prince Ahmed’s role has been more complicated. Since the death of Saudi Arabia’s founder, the crown has passed from one son to the next, always going from brother to brother. Some members of the royal family wanted to see the crown passed to Ahmed, the king’s brother, rather than to MBS, said people familiar with the matter.

Ahmed largely avoided politics since Salman took the crown, but in 2018 he made statements blaming problems in Saudi Arabia on King Salman and MBS when protesters approached him on the street by his London home. Not long after, said a person familiar with the matter, the royal court sent an emissary to London to persuade Prince Ahmed to return to the kingdom.

“Going back to Saudi right now is a big mistake,” a friend of the prince’s recalled telling him at the time. Ahmed, this person said, explained that he wasn’t worried because, while he was on bad terms with MBS, he had a good relationship with his brother King Salman. Ahmed returned shortly after to the kingdom.

While Prince Ahmed has been publicly quiet since then, members of the royal family spoke often in recent months about how he had removed the photographs of King Salman and MBS from the wall of his Majlis, a meeting area for those who come to visit the prince, said a person who participated in those discussions.

MBN has remained out of the public eye since he was shifted from the crown-prince role in a late-night move by MBS. He hasn’t been allowed to leave Saudi Arabia. His public appearances late last year gave some longtime acquaintances the impression that he might be given more freedom. But a few months ago, said a person familiar with the matter, MBS stripped MBN of key staff, including a longtime security guard, replacing them with royal court loyalists.

Ahmed, meanwhile, recently went on a regularly scheduled, weekslong hunting trip. He arrived home in recent days, and people familiar with the matter speculated that the arrests Friday might have been timed for Ahmed’s return.

Write to Summer Said at summer.said@wsj.com and Justin Scheck at justin.scheck@wsj.com