The South Pacific nation of Tonga is in mourning following the death of its king, George Tupou V, at the age of 63.
Prime Minister Lord Tu'ivakano has called for prayers and described this time as a moment when a "black storm cloud" is engulfing the country.
A BBC correspondent says that flags are flying at half mast and civil servants have been told to wear black.
The king died at a hospital in Hong Kong on Sunday with his brother, Crown Prince Tupouto'a Lavaka, by his side.
The cause of his death was not immediately known.
News quickly spread on the internet and was later confirmed in a broadcast on Tongan radio. Many Tongans paid tribute to their monarch.
The national radio station is playing religious music, according to the BBC's Siobhann Tighe in Tonga.
Despite the sense of shock, Tongans have known that the king was unwell. Last year, he had an operation in the US to remove his right kidney.
But he has been fit enough recently to make trips abroad. He even had an audience with the Pope at the Vatican last month.
World leaders have also expressed sadness over the king's death, and tributes have been paid by New Zealand and Australia, which have close ties with Tonga.
New Zealand's Prime Minster John Key said he hoped that the king's efforts working 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 in a statement.
Australian Prime Minister Julia Gillard said the country lost "a great friend", adding that "Tonga's first truly democratic elections, held in November 2010, set the country on a new course".
Tonga is the last Polynesian monarchy, although the king was instrumental in bringing about democratic reform. He had been king since 2006.
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".
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.