Thailand's King Bhumibol Adulyadej, the world's longest reigning monarch, has died aged 88, the palace has confirmed.

Key points: Bhumibol was the only king most Thais have ever known

Bhumibol was the only king most Thais have ever known PM says talk of succession can wait

PM says talk of succession can wait King Bhumibol was widely seen as a unifying force in a fractured political landscape

King Bhumibol was widely seen as a unifying force in a fractured political landscape PM urges vigilance throughout the country to maintain security

"His Majesty has passed away at Siriraj Hospital peacefully," the palace said, adding he died at 3:52pm (local time) on Thursday.

The palace did not give a reason for his death. His reign lasted more than 70 years.

Anxiety about the king's health and the succession has formed the backdrop to over a decade of political upheaval in Thailand that has included two coups.

He was seen as a force for unity, and there have long been concerns that without him the country's divisions could worsen.

Thai Prime Minister Prayuth Chan-ocha said that an heir to King Bhumibol had been designated since 1972, and later announced that the king's son — Crown Prince Maha Vajiralongkorn — had confirmed that he would perform his duties as heir to the throne.

But Mr Prayuth said that talk of King Bhumibol's succession should wait, and that the Crown Prince had asked for time to mourn with the Thai people.

"When the right time comes, the Crown Prince confirmed that he has realised his duty and will continue to perform his duty as the heir to the throne," Mr Prayuth said.

"Let us wait for the right time."

Mr Prayuth, wearing a black suit and tie, urged Thais to love one another and protect "the father's land".

Thailand will officially mourn for one year, and flags will fly at half mast for the next 30 days.

Thailand mourns as King's death leaves political uncertainty

News reaches the crowd at the hospital that the Thai King is dead. ( ABC News: Samantha Hawley )

More than 1,000 people gathered at the hospital where the king had been staying — many of them started to cry as the news of his death broke.

"I feel so saddened by this news," Apinut Jaroonpipatkul, a 25-year-old medical student, said. "He has given so many things to the country."

King Bhumibol was viewed by many in the majority Buddhist nation as a bodhisattva, or holy being who delays entering nirvana to aid the human race.

Most Thais have known no other monarch and his picture is hung in almost every house, school and office across the kingdom.

Members of Thailand's National Assembly met late on Thursday for a special session following news of the king's death — they reportedly stood in silence for nine minutes.

"Today there is nothing. We are preparing for the next steps," Peerasak Porjit, vice president of the National Legislative Assembly, said.

Mr Prayuth however urged vigilance throughout the country to maintain security.

The King had been treated for various ailments over the past year at Bangkok's Siriraj Hospital — his home for much of the past year — and was last seen in public in January, when he spent several hours visiting his palace in the Thai capital.

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On Saturday the King was given haemodialysis — a way of cleansing the blood of toxins, extra salts and fluids — which made his blood pressure drop occasionally, the palace said.

King Bhumibol was just 18 when he took the throne unexpectedly in 1946.

Though he had no real power, since the absolute monarchy ended in 1932, King Bhumibol was seen as a unifying force.

His long reign remained the backdrop to decades of political upheaval, including a coup two years ago when army generals seized power.

With the king's passing, the world's longest reigning living monarch is Queen Elizabeth II, who ascended to the British throne in 1952.

Well-wishers held pictures of Thailand's King Bhumibol Adulyadej at the Siriraj hospital. ( Reuters: Athit Perawongmetha, file )

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