A front-runner in Brazil's presidential election, Jair Bolsonaro, has been stabbed during a campaign rally.
The far-right politician was attacked in the midst of a crowd in the south-eastern state of Minas Gerais.
He underwent surgery for injuries to his intestines and is expected to recover, hospital officials said.
The controversial politician had performed strongly in recent polls, but had outraged many in Brazil with racist and homophobic comments.
Polls suggested he would get the most votes in next month's presidential elections if former President Lula da Silva could not overturn a ban stopping him from standing.
Footage of the incident in the city of Juiz de Fora showed Mr Bolsonaro making a thumbs-up gesture and being held aloft by supporters when he was stabbed with what appeared to be a knife.
He then doubled over with pain and his supporters quickly lowered him to the ground and bundled him into a car.
After the attack, his son Flavio initially tweeted that the wound was "only superficial", but he gave a more sombre assessment two hours later.
"Unfortunately it was more serious than we had expected," he wrote. "He lost a lot of blood, arrived at the hospital with a (blood) pressure of 10/3, almost dead. His condition now seems stabilised. Pray, please!"
Hospital officials later said Mr Bolsonaro had suffered a "deep" and life-threatening stab wound in his intestines and was in a "serious but stable" condition.
He was recuperating well in intensive care after two hours of surgery, but would spend at least a week to 10 days in hospital.
Police said a suspect has been arrested and named him as Adelio Obispo de Oliveira.
Mr Bolsonaro's electoral rivals condemned the stabbing. Fernando Haddad, who was expected to replace Lula da Silva on the Workers Party ticket, said the attack was "absurd and regrettable".
Speaking in the capital Brasilia, President Michel Temer said such an attack was "intolerable" in a democratic state and that he hoped Mr Bolsonaro recovered soon.
"Tolerance is a part of democracy. It is a part of the rule of law," he said.
The president's predecessor, Dilma Rousseff, warned that the attack could not go unpunished "because it must serve as an example so that it doesn't happen to any other candidate".
Mr Bolsonaro's backers saw him as a strong leader who would crack down on crime.
The 63-year-old, who was representing the Social Liberal Party (PSL), was followed by millions of Brazilians on social media, and many refered to him as the "Brazilian Trump".
He supported loosening gun control laws, and was backed by millions of evangelical Christians for his uncompromising anti-abortion stand.
Mr Bolsonaro's outspoken rhetoric and his defence of law and order appealed to many who blamed the left for corruption and the economic crisis.
In 2011, he told Playboy magazine he would be "incapable of loving a gay son" and would rather see such a son "die in an accident".
In 2015, he was fined for saying in a newspaper interview that Congresswoman Maria do Rosario was "not worth raping; she is very ugly".
He was currently being investigated for alleged racism over derogatory remarks he made about Afro-Brazilians.