BANGKOK (Reuters) - King Bhumibol Adulyadej, who died peacefully on Thursday, was the world's longest-reigning monarch, credited with restoring the influence of Thailand's royalty during 70 years on the throne and earning the devotion of many of his subjects.

For the majority of the country's 68 million people, the king was a pillar of stability in rapidly changing times - Thailand embraced industrialization during his reign but also saw its parliamentary democracy punctuated by 10 military coups, the most recent in May 2014.

King Bhumibol, who ascended the throne on June 9, 1946, was seen as a force for unity, and there have long been concerns that the political tensions that have riven Thailand over the past decade could worsen after his death.

That may be less likely under the regime of the leader of the most recent coup, Prime Minister Prayuth Chan-ocha. The former general has held a tight grip on power since toppling the remnants of Thailand's last democratic government in 2014.

"His Majesty has passed away at Siriraj Hospital peacefully," the palace said in a statement on Thursday, adding he died at 15:52 (0852 GMT).

Thailand has been divided for years between the royalist establishment and the red-shirted supporters of exiled former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 coup.

Telecommunications billionaire Thaksin, now in self-exile, built up a powerful patronage network that competed for power and opportunity with Thailand's old-money order.

POOR HEALTH

The king had been in poor health for some time, and has spent most of the past six years in Bangkok's Siriraj hospital.

King Bhumibol was re-admitted in May 2015 and was last seen in public in January, when he spent several hours visiting his Bangkok palace.

The Royal Household Bureau in its statement on Thursday did not give a reason for the king's death. The king been treated for a respiratory infection, a build up of fluid surrounding the brain and a swollen lung in the past few months.

From illuminated billboards in bustling Bangkok to portraits in offices and millions of rural homes, Thailand is festooned with images of the jazz-loving king.

King Bhumibol headed a conservative establishment that still wields considerable power 84 years after the abolition of absolute monarchy.

Born in 1927 in Cambridge, Massachusetts, where his father, Prince Mahidol, was studying medicine, King Bhumibol spent much of his early life abroad, first in the United States and then in Switzerland.

He became king in 1946 after the still unexplained gunshot death of his elder brother, 20-year-old King Ananda Mahidol who was also known as Rama VIII. King Bhumibol returned to Thailand for good four years later to be crowned King Rama IX.

The saxophone-playing King Bhumibol was a celebrity visitor to foreign capitals in the early years of his reign with Queen Sirikit, a distant cousin whom he married in 1950 shortly before his coronation.

The king who acceded to the throne as a young man cut a quite different figure from the somber monarch into which he matured.

Over the years, he was groomed as a national figurehead through civic and ceremonial duties. He undertook a stint in the Buddhist monkhood and developed a keen interest in the environment and rural development.

POLITICS

Though officially above politics, he first started to speak out on political issues in the 1960s against the backdrop of a creeping communist insurgency.

In 1973, he intervened personally after bloodshed in Bangkok when students demonstrated against military rule. He nominated a new prime minister, diffusing the political tension.

Although backing the students then, as a social conservative King Bhumibol was worried about the threat to public order inherent in any people's movement, and three years later he intervened on the side of the military after another bloody putsch.

The king's image as a political truce-maker peaked after bloody clashes in 1992 between pro-democracy protesters and the army. He summoned the protagonists, a former general leading the protests and an army-chief-turned-prime minister, and with the two prostrate before him, ordered them to desist.

His intervention led to the subsequent collapse of military rule.

Often referred to as "Por", the Thai word for father, many Thais looked to him for moral guidance and saw him as a neutral arbiter during their nation's darkest hours.

"We are in the middle," the king said in a 1979 BBC documentary. "One day it would be very handy to have somebody impartial, because if you have in a country only groups or political parties which will have their own interest at heart, what about those who don't have power?"

The king retreated from active political intervention after the events of 1992 in favor of influence wielded through a network of aging generals, judges and bureaucrats on his Privy Council of advisers who helped oversee what some academics view as a "managed democracy", in which the military remained prominent.

The army avoided direct intervention in politics from 1992 until the 2006 coup against Thaksin, a populist telecoms billionaire the military said was corrupt and disloyal to the monarch.

Thailand's monarchy is one of the world's richest, although the value of its assets and the wealth of family members have never been made public.

The Crown Property Bureau, which manages the institutional assets of the monarchy, has stakes in top Thai firms such as Siam Commercial Bank <SCB.BK> and Siam Cement Group <SCC.BK> and extensive land holdings believed to be worth tens of billions of dollars.

The bureau does not publicly disclose its overall income, or detail where the money is spent. The Foreign Ministry insists the bureau's assets are not the king's personal wealth.

Despite the monarchy's wealth, King Bhumibol was the keen proponent of a "sufficiency economy" philosophy - known in Thai as a "just-enough economy", or the idea of moderation and self-reliance, which drew on Buddhist teachings.

REVERED BY MANY

The king was seen as semi-divine by many ordinary Thais, an image bolstered by Thailand's education and legal systems.

"The King shall be enthroned in a position of revered worship and shall not be violated," states the constitution.

Thailand has lese-majeste laws that impose long prison terms for insulting the monarchy. The laws have been enforced harshly as the establishment sought to control new, less deferential political forces and as dissent has found avenues of expression through social media.

Prayuth is a staunch royalist and under his government there has been a surge in prosecutions and tougher sentences for lese-majeste.

King Bhumibol himself said in a 2005 speech that he was open to criticism and those jailed for offending him should be released, but that did not stem the rising number of cases in the troubled years since.

The country faces an uncertain future. The vast majority of Thais have lived only under Bhumibol.

His presumed successor, Crown Prince Maha Vajiralongkorn, 63, has taken a more prominent part in royal ceremonial and public appearances in recent years, but he does not command the same level of devotion as his father.

(Reporting by Bangkok Bureau; Editing by Amy Sawitta Lefevre, Alex Richardson, Bill Tarrant.)