* Al Tayyar Travel founder reportedly detained
* Top newspaper challenges tycoons to bare source of assets
* Saudi newspaper reports no-fly list drawn up
* Purge seen aimed at foiling opposition to reform drive (Recasts, adds quotes, background)
RIYADH, Nov 6 (Reuters) - A campaign of mass arrests of Saudi Arabian royals, ministers and businessmen widened on Monday after a top entrepreneur was reportedly held in the biggest anti-corruption purge of the kingdom's affluent elite in its modern history.
The arrests are the latest in a series of dramatic steps by Crown Prince Mohammed bin Salman to assert Saudi influence internationally and centralise his own power within a hereditary ruling system at home.
The campaign also 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 astonished that a kingdom once obsessed with stability has acquired such a taste for assertive - some would say impulsive - policy-making.
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 series of domestic and international moves asserting the prince's authority.
But outside the kingdom, critics perceive the purge as a further sign of intolerance from a power-hungry leader keen to stop influential opponents blocking his economic reforms or reversing the expansion of his own political clout.
In an article in the Washington Post newspaper, prominent Saudi columnist Jamal Kashoggi applauded the anti-graft campaign but added: "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 princes desire to be seen as a modern, enlightened leader," Kashoggi wrote.
"The buck stops at the leaders door. He is not above the standard he is now setting for the rest of his family, and for the country."
The Saudi stock index initially fell 1.5 percent in early trade, and Al Tayyar Travel plunged 10 percent in the opening minutes after the company quoted media reports as saying board member Nasser bin Aqeel al-Tayyar had been held by authorities.
The stock exchange, or Tadawul, closed effectively flat, up 0.09 percent, rebounding in afternoon trade.
The company gave no details but online economic news service SABQ, which is close to the government, reported Tayyar had been detained in an investigation by a new anti-corruption body headed by Crown Prince Mohammed.
Dozens of people have been detained in the crackdown, which has consolidated Prince Mohammed's power while alarming much of the traditional business establishment. Billionaire Prince Alwaleed bin Talal, Saudi Arabia's best-known international investor, is also being held, officials said at the weekend.
"THE NOOSE TIGHTENS"
The front page of leading Saudi newspaper Okaz challenged businessmen on Monday to reveal the sources of their assets, asking: "Where did you get this?" in bright red text.
Another headline from Saudi-owned al-Hayat warned: "After the launch (of the anti-corruption drive), the noose tightens, whomever you are!"
A no-fly list has been drawn up and security forces in some Saudi airports were barring owners of private jets from taking off without a permit, pan-Arab daily Al-Asharq Al-Awsat said. Among those detained are 11 princes, four ministers and tens of former ministers, according to Saudi officials.
The allegations against the men include money laundering, bribery, extortion and taking advantage of public office for personal gain, a Saudi official told Reuters. Those accusations could not be independently verified and family members of those detained could not be reached.
A royal decree on Saturday said the crackdown was launched in response to "exploitation by some of the weak souls who have put their own interests above the public interest, in order to, illicitly, accrue money".
The new anti-corruption committee has the power to seize assets at home and abroad before the results of its investigations are known. Investors worry that the crackdown could ultimately result in forced sales of equities, but the extent of the authorities' intentions was not immediately clear.
Prince Miteb bin Abdullah was detained and replaced as minister of the National Guard, a pivotal power base rooted in the kingdom's tribes. That recalled a palace coup in June which ousted his elder cousin, Mohammed bin Nayef, as heir to the throne and interior minister.
The moves consolidate Prince Mohammeds control of the internal security and military institutions, which had long been headed by separate powerful branches of the ruling family.
Yet many analysts were puzzled by the targeting of technocrats like ousted Economy Minister Adel Faqieh and prominent businessmen on whom the kingdom is counting to boost the private sector and wean the economy off oil.
"It seems to run so counter to the long-term goal of foreign investment and more domestic investment and a strengthened private sector," said Greg Gause, a Gulf expert at Texas A&M University.
"If your goal really is anti-corruption, then you bring some cases. You dont just arrest a bunch of really high-ranking people and emphasise that the rule of law is not really what guides your actions. It just runs so counter to what he seems to have staked quite a lot of his whole plan to."
Over the past year, MbS has become the ultimate decision-maker on military, foreign, economic and social policies, championing his "Vision 2030" plan which includes subsidy cuts, tax rises, sales of state assets, a government efficiency drive and efforts to spur foreign investment.
The reforms have been well-received by much of Saudi Arabia's overwhelmingly young population, but resented among some of the more conservative old guard, including parts of the Al Saud dynasty frustrated by Prince Mohammed's meteoric rise.
"It's overkill - and overkill in a way that makes it harder to achieve his long-term objectives," said Gause.
The crown prince has also led Saudi Arabia into a two-year-old war in Yemen, where the government says it is fighting Iran-aligned militants, and into a dispute with Qatar, which it accuses of backing terrorists, a charge Doha denies. Detractors of the crown prince say both moves are dangerous adventurism.
The Saudi-led military coalition said on Monday it would temporarily close all air, land and sea ports to Yemen to stem the flow of arms from Iran to Houthi rebels after a missile fired towards Riyadh was intercepted over the weekend. (Additional reporting by Sylvia Westall in Dubai; writing by Stephen Kalin and William Maclean; editing by Andrew Torchia and Mark Heinrich)