The world’s longest-reigning monarch, King Bhumibol Adulyadej of Thailand,  has died after 70 years as head of state.

The 88-year-old king was widely revered but had been in poor health in recent years, making few public appearances.

He was seen as a stabilising figure in a country hit by cycles of political turmoil and multiple coups.

Thailand’s Prime Minister, Prayut Chan-ocha, said Crown Prince Maha Vajiralongkorn will be the new monarch.

In a televised address to the nation, Chan-ocha announced that Thailand would hold a one-year mourning period and that all entertainment functions must be “toned down” for a month.

Describing the king’s death as “the most devastating moment for Thais”, the prime minister said: “He is now in heaven and may be looking over Thai citizens from there.”

In a later statement he urged vigilance, saying national security was a top priority.

“Everyone will need to be alert in every region and throughout the country to ensure safety”, he said.

The king’s death comes as Thailand remains under military rule following a coup in 2014.

King Bhumibol was widely respected across Thailand, and thought of by many as semi-divine.

Hundreds of mourners have gathered outside the Bangkok hospital where he died, stricken with grief at the news.

Many held pictures of the king and wore yellow or pink clothing in his honour.

“How will Thailand live without you father?” cried one distressed man.

“I lost one of the most important people in my life. I feel like I haven’t done enough for him,” said another.

A palace official, speaking to crowds outside the hospital, said the body of the king would be moved to the Temple of the Emerald Buddha in Bangkok on Friday.

Although the prime minister said Crown Prince would become the new monarch, he added that the official proclamation would be made at a later date.

He said the crown prince had confirmed that he would perform his duty as heir to the throne, but had asked for time to mourn his father’s death.

The crown prince, who is 64, is much less well known to Thais and has not attained his father’s widespread popularity. He spends much of his time overseas, especially in Germany.