Colombians expressed horror and revulsion Monday after 33 young children burned to death when their overcrowded bus burst into flames and the driver fled the scene pursued by a bloodthirsty mob.

The driver turned himself over to police as families sought to hunt him down following Sunday's tragedy in which, according to a witness, the bus went up in flames as he refuelled it with a jerrycan.

The children, aged between three and 12, were returning from evangelical church services in the small northern town of Fundacion. The driver lost two of his own children in the fire.

Fundacion Mayor Luz Stella Duran said another 20 were hospitalized. Most of the injured have second- or third-degree burns.

Residents of the town had raced to join firefighters to extinguish the raging blaze, but it was too late.

"We wanted to get out a young girl who was holding out her arms by a window, but her skin was peeling off," witness Jose Guette, 48, told AFP.

"We couldn't do anything but hold our heads and watch the children burn."

'Never forget them'

On Monday, bamboo crosses had been erected along the road where the bus burned, and glass fragments were still scattered on the ground. A woman was on her knees on the asphalt, wailing.

The mayor has declared three days of mourning and banned alcohol consumption.

President Juan Manuel Santos, who rushed to the scene, said an investigation was under way, while many denounced the accident as an example of Colombia's poor enforcement of road safety.

"I am shattered and the whole country is in mourning for the death of these children in this tragedy. We will never forget them," the president said.

Crews pulled small charred bodies from the wreckage and took them to the medical examiner's office in the regional capital of Barranquilla to be identified -- a task which could take days.

The driver gave himself up to police after townspeople hurled rocks at his home. He was due to appear at court to explain how the tragedy unfolded.

The prosecutor's office said the driver lacked a license to drive and will be charged with aggravated manslaughter, as will Manuel Ibarro, a member of the church that hired the bus.

Transport Minister Cecilia Alvarez said the vehicle had lacked the authorization to provide services since 2012.

"It is an illegal vehicle that also lacked transport insurance," Alvarez told RCN radio, adding that the initial investigations are leaning toward "driver recklessness."

The driver apparently used contraband gasoline to refuel the bus, the minister said.

'Tragedy foretold'

An 11-year-old girl who lost two brothers told local media that the driver left the bus to refuel while his young charges remained on board.

"Sparks flew," she recalled. "It was then that the driver went to get water and then he left."

"I broke a window and got my little sister out but I couldn't save my two brothers."

The tragedy became a political issue ahead of Sunday's presidential election, in which Santos is seeking a second term.

Leftist candidate Clara Rojas said the accident "shows the country's backwardness."

"It is not only the driver's irresponsibility. It is also the lack of rigor and presence of the authorities," she said.

Colombians vented their fury on Twitter, one user with the handle @manialolita calling it "another tragedy foretold" while @ALCISERRATO said "the state does not fulfill its duty."

Santos said he had ordered national authorities to intervene more directly in road safety to ensure banned vehicles are not circulating.

He said the government would do "everything we can to ensure such regrettable accidents do not happen again."