Media playback is unsupported on your device Media caption Archive footage of King George Tupou V

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".

At the scene All government ministers, nobles, and civil servants are wearing black as a sign of respect even in the intense heat. Flags are flying at half mast. Most Tongans are starting to do the same, as well as putting the traditional straw mat around their waists. One hair salon is draped in purple and black, again traditional signs of respect, and other buildings will do likewise as the week goes by. The prime minister's broadcast officially announcing that the king had died was on Monday morning but some people had already heard the news the night before through social websites and relatives living abroad. People here seem to be a state of shock even though many knew that the king had been unwell. One woman told me that as soon as she heard the news when she woke up in the morning, she turned to her husband and said: "I miss the king already."

'Saddened'

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.