Jair Bolsonaro, a leading presidential candidate in Brazil, was stabbed on Thursday during a campaign event, though officials and his son said the injury is not life-threatening.
"Patient Jair Messias Bolsonaro recently left. and went to the Albert Einstein hospital", a spokeswoman for the Santa Casa de Juiz de Fora hospital said.
A hospital spokeswoman in the city of Juiz de Fora in Minas Gerais state, where the attack took place, confirmed to Reuters that Bolsonaro had been undergoing surgery but provided no details on his state nor his wounds.
Lula has however been barred from participating by the Supreme Court which disqualified him on Thursday.
Bolsonaro has 22 percent of first-round vote intentions, more than 10 percentage points ahead of his closest rivals, according to an Ibope poll released this week.
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A witness told police the attacker held a knife wrapped up in a shirt.
Police said that the suspect in the attack was arrested - he was mentally unstable 40-year-old Adele Obispo de Oliveira, who stated that "God told" him to stab, Bolsonaro.
Authorities are investigating the suspect's mental health, he added.
His vice presidential running mate, Hamilton Mourao, is a retired general who blamed leftists for the knife attack.
Bolsonaro, a former military man and lawmaker, has been criticized for outbursts deemed racist, mysogynist and homophobic.
Bolsonaro's son, Eduardo, also a congressman, said his father had lost two liters (4.23 pints) of blood from the ordeal.
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Flavio Bolsonaro, a state legislator who is running for a seat in the federal Senate, rejected the idea that his father incited the attack, saying the candidate was engaged in a campaign of ideas.
In a statement, the hospital attending to Mr Bolsonaro said he was in surgery but did not elaborate.
During an event later in the nation's capital, Brazil's President Michel Temer said that "the intolerance that there's been in Brazilian society is unacceptable".
De Oliveira was beaten badly by Bolsonaro supporters after the attack.
One of his campaign pledges has been to legalise the carrying of weapons in order to combat rising violent crime.
He also speaks nostalgically about the country's 1964-1985 military dictatorship and has promised to fill his government with current and former military leaders.
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Bolsonaro, a controversial figure who has enraged many Brazilians for years with divisive comments, has been described as the country's answer to Donald Trump.