Saudi King Salman bin Abdulaziz Al Saud poses for a photo with National Guard Minister Khaled bin Ayyaf and Economy Minister Mohammed al-Tuwaijri during a swearing-in ceremony in Riyadh, Saudi Arabia, November 6, 2017. Saudi Press Agency/Handout via REUTERS

By Stephen Kalin and Reem Shamseddine

RIYADH (Reuters) - A campaign of mass arrests of Saudi Arabian royals, ministers and businessmen expanded on Monday after a top entrepreneur was reportedly detained in the biggest anti-corruption purge of the kingdom's affluent elite in its modern history.

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.

"The kingdom is at a crossroads: Its economy has flatlined with low oil prices; the war in Yemen is a quagmire; the blockade of Qatar is a failure; Iranian influence is rampant in Lebanon, Syria and Iraq; and the succession is a question mark," wrote ex-CIA official Bruce Riedel.

"It is the most volatile period in Saudi history in over a half-century."

The crackdown has drawn no public opposition within the kingdom either on the street or social media. Many ordinary Saudis applauded the arrests, the latest in a string of domestic and international moves asserting the prince's authority.

For more news videos visit Yahoo View.

But abroad, critics perceive the purge as further evidence of intolerance from a power-hungry leader keen to stop influential opponents blocking his economic reforms or reversing the expansion of his political clout.

Prominent Saudi columnist Jamal Kashoggi applauded the campaign, but warned: "He is imposing very selective justice."

"The crackdown on even the most constructive criticism - the demand for complete loyalty with a significant 'or else' - remains a serious challenge to the crown prince’s desire to be seen as a modern, enlightened leader."

"The buck stops at the leader’s door. He is not above the standard he is now setting for the rest of his family, and for the country," he wrote in the Washington Post.

ACCOUNTS FROZEN

The Saudi stock index initially fell 1.5 percent in early trade but closed effectively flat, which asset managers attributed to buying by government-linked funds.

Al Tayyar Travel <1810.SE> plunged 10 percent in the opening minutes after the company quoted media reports as saying board member Nasser bin Aqeel al-Tayyar had been detained in the anti-corruption drive.

Saudi Aseer Trading, Tourism and Manufacturing <4080.SE> and Red Sea International <4230.SE> separately reported normal operations after the reported detentions of board members Abdullah Saleh Kamel, Khalid al-Mulheim and Amr al-Dabbagh.

Saudi banks have begun freezing suspects' accounts, sources told Reuters.

Dozens of people have been detained in the crackdown, which have alarmed much of the traditional business establishment. Billionaire Prince Alwaleed bin Talal, Saudi Arabia's best-known international investor, is also being held.

The attorney general said on Monday detainees had been questioned and "a great deal of evidence" had been gathered.

"Yesterday does not represent the start, but the completion of Phase One of our anti-corruption push," Saud al-Mojeb said. Probes were done discreetly "to preserve the integrity of the legal proceedings and ensure there was no flight from justice."

Investigators had been collecting evidence for three years and would "continue to identify culprits, issue arrest warrants and travel restrictions and bring offenders to justice", anti-graft committee member Khalid bin Abdulmohsen Al-Mehaisen said.

"THE NOOSE TIGHTENS"

The front page of leading Saudi newspaper Okaz challenged businessmen to reveal the sources of their assets, asking: "Where did you get this?"

Another headline from Saudi-owned al-Hayat warned: "After the launch (of the anti-corruption drive), the noose tightens, whomever you are!"

Story continues