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Thailand is celebrating the 85th birthday of King Bhumibol Adulyadej, the world's longest-reigning monarch.

King Bhumibol emerged from the hospital where he has spent the last three years to orchestrate the celebrations and deliver a brief speech to the 200,000-strong crowd that gathered in Bangkok to catch a rare glimpse of him.

Sitting on a throne on a balcony of Dusit Palace, the monarch called for unity and stability as Thailand faces great political turmoil.

"Your best wishes and goodwill to each other is an important key to making unity happen," the king said.

"If Thai citizens still hold this virtue (harmony) in their hearts, there is hope that in whatever the situation, Thailand would surely get through it safely and stably."

The speech was welcomed by a "long live the king" roar from the crowd. Many fainted due to emotion, as well as the high heat and humidity.

The king was flanked by his son Crown Prince Vajiralongkorn, and other members of the royal family. However his 80-year-old wife Queen Sirikit, who suffered stroke-like symptoms in July, missed the ceremony on doctors' advice.

King Bhumibol ascended to the throne in 1946, six years before Britain's Queen Elizabeth II, who is second in the list of the world's longest-reigning monarchs.

The monarch is still a much revered figure in Thailand, and his venerability is protected by severe lese majeste laws which carry sentences of up to 15 years in jail.

King Bhumibol was hospitalised in 2009 for a lung inflammation, and has rarely appeared in public since.

The king's health troubles have triggered speculation on the future of the monarchy. Crown Prince Vajiralongkor enjoys a much lower popularity rating and the royal family's traditionally apolitical stance has recently been compromised.

Current Prime minister Yingluck Shinawatra and her brother, former prime minister Thaksin, have often been accused of plotting against the monarchy. However Thaksin Shinawatra's supporters believe the royal family is behind the coup which overthrew his government in 2006.