King Chulalongkorn reigned for 42 years between 1868 and 1910, during which time he embarked on radical institutional reforms that laid the foundations of the modernisation of what was then known as Siam.

King Bhumibol Adulyadej will celebrate the 60th anniversary of his accession to the throne on Friday.

Historically, this is quite remarkable, not only for Thailand, but for King Bhumibol is also the world's longest-reigning monarch.

In royal tradition, a grand celebration takes place every 25 years, starting with the Silver Jubilee. This is followed by golden and diamond jubilees.

His Majesty the King celebrated the Silver Jubilee of his accession to the throne in 1973 and the Golden Jubilee in 1996. Both Queen Elizabeth II of England and the late Prince Rainier of Monaco have also celebrated golden jubilees.

HM the King has agreed to allow celebrations to be held in his honour on Friday as requested by the Royal Thai Government. These celebrations have been titled "The 60th Anniversary Celebrations of His Majesty the King's Accession to the Throne".

Kings, queens and royal re-presentatives from 29 countries have been invited to participate in the celebrations, which will take place between Thursday and Tuesday, and will include traditional religious and royal ceremonies.

One of the highlights will be the Royal Barge Procession, which will showcase the country's rich cultural heritage.

The celebrations will begin on Thursday with a religious ceremony paying homage to the King's ancestors of the Chakri dynasty. The ceremony will be held in the extended part of the Chakri Throne Hall within the Grand Palace.

Their Majesties the King and Queen will take part in this event by lighting candles and joss-sticks and worshipping Buddha images and the remains of their ancestors.

The King will present decorated fans used in religious ceremonies to a hierarchy of 26 monks. The monks will then chant a Buddhist prayer. The King and Queen will light the candles and joss-sticks and worship again. After prayer, Their Majesties will offer the basic necessities and saffron robes to the monks before ending the ceremony.

A grander ceremony is reserved for Friday, the day the King acceded to the throne 60 years ago in 1946. Court officials are preparing a religious ceremony to pay homage to the great Siamese kings of the past who founded their kingdoms. The Siamese kings to be honoured are Phokhunsri Indhrathit of Sukhothai, Somdej Phrachao U-thong of Ayutthaya, King Taksin the Great of Thonburi and King Yodfa of Bangkok.

This grand event will take place at the Chakri Throne Hall. Their Majesties will then proceed to the Sala Thai behind the throne hall to light candles and joss-sticks to pay homage to these Siamese kings. The chief Brahmin priest will conduct the worship, after which a court astrologer will bang a gong. Another Brahmin priest will make a sound by blowing into a large conch shell, after which an orchestra will perform. Their Majesties will then offer the basic necessities to the senior monks.

Later, Their Majesties will grant an audience to the members of the Royal Family, members of the Privy Council, and other descendants of the Chakri Dynasty in the throne hall.

HRH Crown Prince Maha Vajiralongkorn will open a vase containing flowers, candles and joss-sticks and offer blessings to the King on behalf of the Royal Family. The prime minister, the president of the Senate, the president of the Supreme Court, the Cabinet, diplomats and other members of Parliament, senior civil servants and judges will also be present.

On Saturday, the King and Queen will proceed again to the Chakri Throne Hall, where a ceremony will be held to honour the most senior monks. Ten monks will offer a prayer of blessing. A total of 64 monks will be promoted to higher ranks. Then 99 monks will offer a prayer of blessing to the King.

On Monday, Their Majesties will proceed to the Ananda Samakhom Throne Hall to greet the kings, queens and royal representatives invited from 26 countries. The diplomats and representatives of international agencies will also be present.

In the evening, Their Majesties and their guests will proceed to the Royal Navy Conference Hall

to watch the Royal Barge Procession.

The celebrations will end on Tuesday when Their Majesties attend a gala dinner with the kings, queens and royal representatives from the 26 countries at the Chakri Throne Hall.