Brazilian far-right presidential candidate Jair Bolsonaro is undergoing surgery for a serious injury to his liver, which was perforated when an attacker stabbed him during a campaign rally on Thursday for next month's election, GloboNews TV reported.
"His internal wounds were grave and put the patient's life at risk", said Luiz Henrique Borsato, the doctor who operated on Bolsonaro.
A suspect, identified by authorities as 40-year-old Adelio Bispo de Oliveira, was arrested within seconds.
Video footage showed Bolsonaro atop supporters' shoulders, clutching his stomach and grimacing after the assault.
Flavio Bolsonaro, son Jair Bolsonaro, gestures outside Santa Casa hospital where his father was being treated in the southeastern city of Juiz de Fora on Friday.
Bolsonaro, whose trademark pose at rallies is a "guns up" gesture with both hands like pistols, has said he would encourage police to kill suspected drug gang members and other armed criminals with abandon.
Bolsonaro was taken to hospital in the town of Juiz de Fora, about 125 miles north of Rio de Janeiro, after he was stabbed by a man while he was being carried by a crowd of supporters.
Brazil's stocks and currency extended gains after the stabbing as traders bet the incident could boost support for Bolsonaro, who has tapped a University of Chicago-trained banker as his main economic adviser.
Brazilian far-right congressman Jair Bolsonaro was stabbed in the abdomen during a campaign event Thursday.
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In an article previous year calling him "Brazil's Donald Trump", The Economistdescribed Bolsonaro as a "religious nationalist and former army captain, he is anti-gay, pro-gun, and an apologist for dictators who tortured and killed Brazilians between 1964 and 1985".
Bolsonaro is one of the most controversial candidates in the current electoral round because he is a defender of Brazil's 1964-1984 military dictatorship.
"This episode is sad", President Michel Temer told reporters, according to the AP. Bolsonaro is well ahead of other presidential hopefuls in the opinion polls.
Environmentalist candidate Marina Silva said the attack must be investigated and the perpetrators punished.
Political violence is common at state and city levels in Brazil, most recently with the high-profile assassination of a Rio de Janeiro city councilwoman.
Other presidential candidates quickly denounced the stabbing and many of them made a decision to suspend their campaign events Friday.
Fernando Haddad, who will likely be the leftist Workers Party presidential candidate, said the stabbing was a "shame" and a "horror".
The attack ocurred as the candidate was moving slowly through a large crowd. A medical team is evaluating whether Mr Bolsonaro can be transferred to another hospital in Sao Paulo.