Jair Bolsonaro, Brazil's front-running far-right presidential candidate, underwent emergency surgery on Thursday after he was stabbed during a campaign event.

The 63-year-old lawmaker has "a wound in the abdomen that has been caused by a sharp instrument," said the Santa Casa hospital in the southeastern city of Juiz de Fora, describing his condition as "stable."

In a press conference after the surgery, doctors said the lawmaker had "three serious bowel perforations" that caused internal bleeding, adding that they had performed a colostomy and he was now stable.

Earlier, one of the candidate's sons, Flavio Bolsonaro, had announced on Twitter that his father's wounds were "superficial", but he later wrote: "Unfortunately, it's more serious than we thought."

"The perforation reached part of his liver, lung and intestine," Flavio Bolsonaro wrote. "He lost a lot of blood, arrived at the hospital ... almost dead. He appears to have stabilised now. Please pray!"