The group of 35 monks and five lay followers were walking along a road in Mukdahan province during a pilgrimage when the incident occurred.

Footage posted online shows saffron robes and belongings scattered by the roadside, and a wrecked vehicle. Five of the monks died at the scene and four others later in hospital, according to police.

They said the boy took the pickup without permission before losing control and crashing into the monks, according to Agence France-Presse.

Police Major General Pairoj Thaiphutsa said they were still determining the legal process for the boy going forward.

"The suspect is a child. The vehicle has been taken for forensic examination to determine the cause," he said.

Later, Thaiputsa told BBC Thai the boy was still being cared for by a team of officials, including his guardians and his doctor.

Preliminary information indicates that he is a child with special needs, but no further details are available yet, Thaiputsa said.

Mukdahan Hospital has appealed for urgent blood donations to help the injured monks.

Three monks remain in a critical condition and at least five others were seriously hurt. More are being treated for lighter injuries, health authorities said.

Phra Sompong, who was a monk in the group, said he was chanting the meditation mantra "Buddho, Buddho" before the crash happened.

"I saw a boy driving a pickup truck approaching ... then suddenly the truck hit at full speed and crashed us," he said in a video posted online by local rescue workers.

"Luckily another monk and I managed to jump out of the way in time.

"The first nine monks in line survived, but others who were hit were thrown into the air."

Buddhist monks are highly respected in Thailand as they are entrusted with preserving and passing on the Buddha's teachings.

Public processions are not uncommon, with members of the public often giving monks alms of goodwill - such as food and basic necessities - as a sign of respect.

Police further said initial inquiries have found that no one was home before the incident except the child suspect, who did not go to school because he was unwell.

When the boy's guardians realised the car was missing they notified police.

Mukdahan provincial governor Worayan Bunnarat said the case should serve as a wider warning for road safety in Thailand.

"We've been very strict on road safety in recent years. This case should be a lesson not just for our province, but for the public in general when it comes to preventing road accidents," he said.

"I think everyone involved, especially parents, needs to help, because no one wants something like this to happen."