A 13-year-old boy has been miraculously rescued after falling into a river of sewage in Los Angeles, getting swept away and spending more than 12 hours in the city’s toxic and maze-like underground sewer system.

Jesse Hernandez had been playing with other children on wooden planks over an access portal to the sewer system during a family outing at a Los Angeles park on Sunday.

When a plank broke, Jesse fell about eight metres and landed in fast-moving sewage, said Brian Humphrey, a spokesman for the Los Angeles Fire Department.

The other children immediately notified adults, who called 911, initiating a frantic and exhaustive 12-hour search of labyrinthine underground pipes using cameras propped on flotation devices.

Firefighters working where a 13-year-old boy fell through. Photo: AAP

Rescuers finally found Jesse after seeing images of handprints on a sewage pipe. A sanitation crew rushed to the area and opened a manhole.

“The first thing they heard was ‘Help!'” Adel Hagekhalil, assistant general manager of the sanitation department, said.

The crew lowered down a hose to Jesse, who was about three metres deep in the pipe.

“He caught onto the hose and was reeled back up,” Mr Hagekhalil said.

Rescuers gave him immediate medical attention, including hosing him down to get rid of the sewage and cleaning out his eyes and nose with sterile saline, Mr Humphrey said.

Jesse immediately asked for a cellphone to call his family. A worker handed him a phone, and he called his mother.

“He was happy, the family was happy,” Mr Hagekhalil said, adding that rescuers were thrilled at the outcome.

The odds of someone falling into such a pipe and surviving are slim. The odds were not in his favour, and many would call it miraculous. It’s obvious he’s not a quitter.”

After an accident like Jesse’s, rescuers say the likelihood of someone being found safe diminishes by the hour.

“They never gave up hope,” Mr Hagekhalil said. “They wanted to bring Jesse back to his family.”

About 731 metres of pipe had been inspected when rescuers found Jesse less than a mile from where he disappeared.

In addition to the massive rescue effort involving more than 100 people, Humphrey credited Jesse for his survival.

Not only did he survive getting swept through sewage moving at 24km/h, he managed to find a pocket of breathable air and hang on until he was found, authorities said.

“Any subterranean location, particularly one that involves waste, can produce toxic gases – methane, hydrogen sulfide – so breathable air is a key element,” Humphrey said.

-AAP