King George Tupou V of the South Pacific nation of Tonga has died at the age of 63.
He died at a hospital in Hong Kong with his brother, Crown Prince Tupouto'a Lavaka, at his side.
The BBC's Siobhann Tighe in Tonga says a period of mourning has begun and a state funeral is being organised.
Tonga is the last Polynesian monarchy, although the king was instrumental in bringing about democratic reform. He had been king since 2006.
The cause of King Tupou's death on Sunday was not immediately known.
Our correspondent says the news has shocked Tongans. National radio has been playing hymns and religious music, she adds.
People across the 170-island archipelago were able to vote for their first popularly-elected parliament in November 2010, ending 165 years of feudal rule.
King Tupou said on the eve of the vote he was granting his executive powers to the cabinet and parliament, adding that "in future the sovereign shall act only on the advice of his prime minister".
News of his death quickly spread on the internet and was later confirmed in a broadcast on Tongan radio.
Many Tongans paid tribute to their monarch.
"I am saddened by the news, our beloved King of Tonga has passed away... our country has gone through major losses in the past couple years," Tessi Leila Tolutau was quoted as saying by the New Zealand Herald newspaper.
In September 2006, King Tupou succeeded his father King Taufa'ahau Tupou IV, who died after a long illness.
But deadly riots fuelled by demands for reform delayed his coronation until August 2008.
The Oxford-educated monarch was a bachelor. He named his brother heir apparent.
King Tupou was known for his fondness of military uniforms and for being driven around in a London taxi.