A solemn but colour-splashed procession streamed through Bangkok's historic heart on Thursday as Thailand bade farewell to King Bhumibol Adulyadej in an elaborate, ritual-laden funeral that gripped a nation mourning the loss of its chief unifying figure. 

Some 300,000 black-clad mourners packed the streets from early Thursday, many weeping and prostrating themselves on the ground as a golden chariot carrying the royal urn slowly snaked through the blazing heat.

Pipers, drummers and soldiers in a dazzling array of costumes joined Buddhist monks, Brahmin priests and the new King Maha Vajiralongkorn as the procession made its way to the spectacular funeral pyre.

King Vajiralongkorn led royals, junta leaders and foreign dignitaries up the central tower to lay sandalwood flowers at the pyre ahead of the cremation, scheduled for 10pm (1500GMT), to a haunting accompaniment chanted by traditional singers. 

The $90 million funeral drew a "Who's Who" of Thai power -- royals, generals and establishment figures -- as well as scores of foreign guests including Great Britian's Prince Andrew and Japan's Prince Akishino and Princess Akishino.

For the public, the lavish affair was a chance to say a final goodbye to a monarch cherished as the "father of the nation".

Bhumibol, who was crowned in 1950, towered over decades of Thai history before his death last October aged 88 seeded uncertainty in a country ruled by a divisive junta.

A brew of palace propaganda and a harsh lese majeste law burnished the king's reputation throughout his reign.

But Bhumibol's intimate connection with his subjects was on display Thursday.

"He was perfect. He helped the country and Thai people so much. Seventy million Thai people are united in their love for him," said 65-year-old Wacharadej Tangboonlabkun, who like most Thais knew no other monarch before Bhumibol's death.

- Tumultuous reign -

The death of a figure of constancy in a politically combustible country has dipped the kingdom into uncertainty.

"There's no more a father who only gave to his children," 47-year-old mourner Kingkan Kuntavee told AFP. 

For much of Bhumibol's long reign, Thailand remained stuck on a carousel of violent protests, short-lived civilian governments and coups.

Political turmoil threw up a supply of junta leaders and prime ministers, but all lacked Bhumibol's moral capital with the Thai people. 

He left behind one of the world's richest monarchies, one that stands at the apex of one of Southeast Asia's most unequal societies.

Deference towards the monarchy -- and the social elites it underpins -- is a given in Thailand.

Ahead of the processions, palace aides shuffled on their knees in the presence of the new king, as monks in orange robes chanted Buddhist prayers.

The new king, who wore full military regalia, will be crowned after his father is laid to rest.

He has yet to win the same affection among the Thai public as his father, whose image was carefully curated by palace PR, cementing a reputation as austere, benevolent and incorruptible despite the fast-changing times.

Thailand's royal defamation law shields the monarchy from criticism and scrutiny, carrying 15-year jail sentences for each charge.

That law makes independent analysis and frank public debate about the monarchy impossible inside Thailand.

In effect it means the monarchy "has monopolised the way the Thai public can think about its own political story," historian David Streckfuss told AFP.

The ruling junta has jailed record numbers of people under the law since seizing power in a 2014 coup. 

Aged just 18 when he ascended the throne, the US-born Bhumibol became the fulcrum of the palace and was the world's longest-reigning monarch until his death. 

The crown flourished with heavy US backing as Washington sought a bulwark against the spread of Communism across Southeast Asia.

Thais have donned black for much of the last year in a remarkable outpouring of grief, which officially ends on October 30. 

They are expected to return to colourful clothes at the conclusion of the mourning period, which celebrates the king's ascent to Mount Meru, the centre of the universe in Buddhist, Hindu and Jain cosmology.