Tonga's King George Tupou V, who gave up most of his powers to bring a more democratic government to his Pacific island nation, has died at a Hong Kong hospital.
Prime minister Lord Siale'ataonga Tu'ivakano said the king had died at 3pm yesterday, local time. No cause of death was given.
The premier said the king's younger brother and heir to the throne, Crown Prince Tupouto'a Lavaka, was at the king's side when he died.
The king had a liver transplant last year and suffered other health problems, according to Tongan media reports. He had reigned over the island nation of 106,000 since his father, King Taufa'ahau Tupou IV, died in 2006.
The father had long resisted ceding any powers of Tonga's absolute monarchy during his four-decade rule. But after his death, rioters unhappy with the pace of reforms took to the streets and destroyed the centre of the capital Nuku'alofa.
Against that backdrop, the new king delayed his official coronation until 2008 while he put together the framework for sweeping political reforms. Three days before the coronation ceremony, Tupou announced he was ceding most of his executive powers to a democratically elected parliament. The king remains head of state, and some parliament seats are reserved for nobility.
While the parliament is now responsible for much of the day-to-day running of the country, the king retains the right to veto laws, decree martial law and dissolve the parliament.
New Zealand Prime minister John Key said he hoped the king's work leading Tonga towards democracy would continue.
"He believed that the monarchy was an instrument of change and can truly be seen as the architect of evolving democracy in Tonga. This will be his enduring legacy," Mr Key said.
Tupou V did allow himself an elaborate coronation, a five-day-long affair that included roast pig feasts, tribal rites and British-style pomp. Its £1.6 million price tag put a heavy strain on the impoverished nation.
He will be remembered by many for his throwback fashion choices - which included wearing, at times, a top hat and even a monocle. He studied at King's College in Auckland, New Zealand, and in Britain.
Pesi Fonua, publisher of the Tongan news website Matangi Tonga, said the king also campaigned for technologies such as mobile phones and the internet, and made some enemies among conservative Tongans for his efforts to make the economy more market-driven and open to foreign competition and investment.
The prime minister declared that the royal family and entire nation was in mourning. Mr Fonua said he ended his address with a Tongan expression meaning "the sun has set".