Weeping relatives and friends mourned the 20 children and three teachers who died in a horrific bus fire in Thailand at a funeral on Thursday.

White and gold coffins lined the hall at Wat Khao Phraya Sangkharam School in Lan Sak, a small town in central Thailand. Relatives placed foods, clothes and toys on some of them.

A representative of the Thai royal family poured water on the coffins - a Buddhist funeral rite that symbolises purification - and placed wreaths.

Police have not determined the cause of Tuesday's fire, which happened while the group was on a field trip. The bus driver, who turned himself in, has been charged with reckless driving causing deaths and injuries.

Nineteen children and three teachers are reported to have survived the fire. Sixteen of them are being treated in hospital for injuries. The ages of the children on board remain unclear, but the school caters for pupils between three and 15 years old.

Hundreds of people, including several cabinet ministers, attended the mass funeral. Among them was Education Minister Permpoon Chidchob, who handed out financial assistance to the victims' families.

Authorities have carried out "merit-making ceremonies" or Buddhist rituals for those who lost their lives, the minister said.

"We made merit and prayed also for the students who are still at the hospital for their speedy recovery," he said.

Some residents and monks in Lan Sak stayed up on Wednesday to receive the bodies from a mortuary in the capital, Bangkok.

A funeral procession was held earlier on Thursday, which saw families weeping as they clutched framed photographs of the victims.

Buddhist prayers for the victims will go on for the next four days, followed by a royal cremation ceremony next week ordered by Thailand's King Maha Vajiralongkorn.

Across Thailand, teachers and students laid white flowers in their schools to mourn those who died in what is believed to be Thailand's deadliest traffic accident in a decade.

The bus driver had told investigators the vehicle lost balance and scrapped a concrete barrier. Sparks of the collision could have set off the blaze, deputy regional police chief Chayanont Meesati said on Wednesday.

The bus, which was fuelled by compressed natural gas, had 11 gas canisters although it was permitted to install only six. The company that owned that bus told local broadcaster ThaiPBS that the vehicle had passed safety standards.

The Thai government has since ordered an inspection of all passenger buses equipped with compressed natural gas.

Tael Narach, the grandmother of a six-year-old victim told Reuters news agency she "fainted right away" on Tuesday when she got news of the fire from her son-in-law.

"I want justice and support (to get justice) for my grandchild," said Tael, 69.