The wail of conch shells and the chanting of monks opened a celebration honoring Bhumibol Adulyadej, the hugely popular king who on Saturday became the longest-reigning monarch in Thailand's 750-year history.

The king's 42 years and 23 days on the throne eclipsed the tenure of King Chulalongkorn, a visionary monarch who ruled from 1868 to 1910, when Thailand still was an absolute monarchy.The king and Queen Sirikit lit candles and incense sticks before- the ashes of King Chulalongkorn to begin the four days of religious and royal celebrations. Monks chanted and conch shells sounded during the ceremony.

The king on Friday signed a decree to free or reduce the sentences of about 50,000 prisoners charged with minor crimes. Such royal amnesties are issued periodically to mark special occasions.

The ceremonies in the Grand Palace - a walled compound of glittering spirals and imposing structures - opened with the presentation of special "prayer fans" to 44 senior Buddhist monks.

Prime Minister Prem Tinsulanonda, Cabinet ministers and senior military officers were at the palace for the rites along with members of the royal family.

Bangkok buildings have been festooned with lights and decorations for several days and a display of 29,999 fireworks is planned. King Bhumibol is the ninth monarch of the current Chakri dynasty, and nine is generally considered an auspicious number by Thais.

Public festivities, including movies, concerts and folk dances, have been scheduled throughout the country.

The royal family is also to visit Ayuthaya, where they will pay respects to the spirits of kingly ancestors of the old capital, which was sacked by the Burmese in 1767.

The royal seat subsequently was moved to Bangkok, where the Chakri dynasty was founded in 1782 by Rama I.

Rama IV, popularized in the West in the book and movie "The King and I," began changing traditional concepts of absolute rule and introduced Western know-how to the country. Rama V reorganized the state machinery, opened Thailand to further Western influence but adroitly kept colonial powers at bay.

Bhumibol, who turned 60 in December during another nationwide celebration, was born in Boston, where his father studied medicine.

The Swiss-educated prince assumed the throne upon the still-mysterious death of his brother in 1946. He gradually restored some of the power and prestige the monarchy lost in 1932 when a bloodless revolution ended its absolute rule.

The king and his wife were a hit during a series of world tours in the 1960s. The multi-talented monarch frequently shed his normal seriousness to pick up a saxophone and join in jazz jam sessions.

In the 1970s and '80s, he and members of his family turned their attention to the country's rural poor, initiating a network of royal projects in irrigation, reforestation, agriculture and the replacement of opium by other cash crops.

The king would spend up to nine months of each year in the countryside, away from the opulent palaces and elaborate ceremonies in Bangkok.

The king has stepped in on several occasions of national crisis, including a 1973 student revolt against a trio of military strongmen. The bloodshed was stopped and the trio exiled. He is widely believed to wield considerable behind-the-scenes power and is looked upon by Thais as a factor of stability in a modern history punctuated by frequent military coups and factionalism.