Thousands of black-clad mourners are gathering in the historic royal quarter of Thailand's capital to bid farewell to King Bhumibol Adulyadej.

Some had been waiting for one or two days in relentless tropical heat and torrential downpours to enter the cordoned-off area that will be the focal point for five days of ceremonies and processions including Bhumibol's cremation on Thursday evening.

Police tried to calm occasional flare-ups of tension among mourners waiting in the hot sun. There were accusations of queue-jumping and sharp exchanges between some of the mourners.

Volunteers were handing out water as the crowds slowly moved through security checks.

Around 250,000 people are expected to line streets for the elaborate gilded processions that will be broadcast live.

The funeral for the monarch who was adored as a unifying symbol throughout his seven-decade reign officially begins mid-afternoon on Wednesday with a Buddhist merit-making ceremony in the throne room of the Dusit palace.

Mourner Banterng Saeuong, 55, said "I was born in the reign of King Rama 9", as Bhumibol is also known. "This is the most important event in my lifetime.

"I am happy to stand in the rain or sit under the sun."

Bhumibol, who was the world's longest reigning monarch, died aged 88 on October 13 last year, sparking a national outpouring of grief and a year of official mourning.

Thais are known for their reverence of Bhumibol, which palace officials assiduously cultivated over his 70-year reign, and the funeral will be an intensely sombre event.

The influence and status of Thailand's monarchy was revived during his reign and he earned genuine affection for high-profile visits to remote parts of the countryside and efforts to improve the lot of the country's poorest.

Yuwadee Tyler said she had travelled from Hobart in Australia, where she has lived for more than a decade.

"When I know my king is passed away, my heart is broken," she said. "I am so glad to be here."