Right-wing presidential front-runner Jair Bolsonaro was stabbed in the stomach in a knife attack while campaigning in southeastern Brazil, his son Flavio said on Thursday.
Flavio Bolsonaro – who initially described his father's wounds as superficial – later tweeted that the 63-year-old suffered injuries to the liver, lung and intestine and had "lost a lot of blood."
"Sadly it is more serious than we thought," he posted, as his father underwent emergency surgery at a nearby hospital in Juiz de Fora, a town of 700,000 people in Minas Gerais state.
The hospital itself, quoted by the G1 website, said Bolsonaro suffered a perforated intestine, and described his condition as "stable."
"Thank God it was barely superficial and he's fine," Flavio Bolsonaro had initially posted on Twitter.
Images shared on social media and television showed Bolsonaro being carried on the shoulders of a throng of supporters before a man lunges at his stomach.
The apparent attacker is then grabbed by several Bolsonaro supporters before the video cuts out. Other images show the presidential candidate being carried to safety.
A military police spokesman told AFP a 40-year-old man had been arrested in connection with the assault. The suspect was carrying a knife wrapped in fabric, the spokesman said.
Bolsonaro, a 63-year-old former Army captain, was taken to a nearby hospital in the town of Juiz de Fora, about 125 miles (200 kilometres) north of Rio de Janeiro, police said.
With jailed former president Luiz Inácio Lula da Silva ruled out of the October 7 election, the latest polls from the Ibope Institute put Bolsonaro in a clear lead with 22 percent compared to 12 percent each for environmentalist Marina Silva and centre-left runner Ciro Gomes.
One of his campaign pledges has been to legalise the carrying of weapons in order to combat rising violent crime.
Despite being a long-serving member of Congress, he has successfully presented himself as an outsider, untouched by the corruption scandals engulfing so much of the political elite.
Yet he's also courted controversy with comments about homosexuals, women and victims under the 1964-85 military dictatorship. Sometimes described as Brazil's Donald Trump, he has a huge social media following of 8.5 million people.
Perhaps the message that carries furthest is Bolsonaro's push for a harder crackdown on crime – in a country where police are already often engaged in low-level wars against gangs. About 64,000 people die in homicides in Brazil every year.