Hundreds of thousands of black-clad mourners crowded Bangkok's historic quarter today as the cremation of the revered King Bhumibol Adulyadej officially began with Buddhist religious rites and ceremonial candle-lighting by saffron-robed monks.

Many of those lining the Thai capital's streets had slept overnight on pavements near the Grand Palace to capture vantage spots for the funeral procession that wended past buildings draped in yellow marigolds.

King Bhumibol, also known as King Rama IX, died last October aged 88 after ruling for seven decades. He played a pivotal role in maintaining stability during years of political upheaval and rapid development.

At the start of the ceremony, officials dressed in blue and orange removed a symbolic golden urn from the Dusit Maha Prasart Throne Hall and carried it through the streets to a 50-metre high royal crematorium in a nearby square.

The king's body was moved to the cremation area on Wednesday night from the palace, where it had been lying in state since his death.

New King Maha Vajiralongkorn presided over religious chanting in the afternoon prior to the cremation at the royal crematorium, which features nine spires to mark the reign of his father, the ninth member of Thailand's Chakri Dynasty.

One of the late king's daughters, Princess Maha Chakri Sirindhorn, lit candles inside the crematorium.

Thailand's supreme patriarch, Somdet Phra Maha Muniwong, led the religious ceremony, urging Thais to follow Buddhist teachings and "look beyond sadness".

Royals from Bhutan, Britain, Japan, Lesotho, the Netherlands, Norway, Spain and Sweden, among others, attended the funeral ceremony.

Officials in charge of the ceremony said about 110,000 people had gathered near the cremation area, with 200,000 more nearby.

A sum of $90 million has been set aside for the funeral, the likes of which has never been seen in Thailand, officials have said, even though King Bhumibol was portrayed as a frugal man despite being one of the world's wealthiest monarchs.

Elsewhere in the Thai capital and around the country, 85 smaller replicas of the royal crematorium and more than 870 pavilions were built for mourners to lay sandalwood flowers and pay their final respects.

Sites to offer sandalwood flowers were also set up in 94 countries to give overseas Thais a chance to say good-bye.