Brazil's right-wing presidential frontrunner Jair Bolsonaro was seriously injured Thursday when he was attacked with a knife while campaigning in the country's southeast, his son said.

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."

Images shared on social media and Brazilian television showed Bolsonaro being carried on the shoulders of a throng of supporters, before a man lunges at his stomach.

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.

Social media footage showed the apparent attacker being grabbed by several Bolsonaro supporters before the video cuts out.

Other images showed the presidential candidate -- a former army captain -- being carried to safety and Bolsonaro's son initially said his father was "fine."

With jailed former president Luiz Inacio 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 center-left runner Ciro Gomes.

One of his campaign pledges has been to legalize 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.

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 every year.

Yet he's also courted deep controversy with comments attacking women and sexual minorities, as well as for praising the country's 1964-1985 military dictatorship.

Sometimes described as Brazil's Donald Trump, Bolsonaro has a huge social media following of 8.5 million people.

The only Brazilian politician currently more popular is the leftist ex-president Luiz Inacio Lula da Silva -- who is in prison for corruption and has been banned from competing in October's election.

Top image: In this file picture taken on September 5, 2018 Brazilian right-wing presidential candidate Jair Bolsonaro speaks during a campaign rally in the district of Ceilandia in Brasilia. Brazil presidential hopeful Bolsonaro was hurt in a knife attack during a campaign rally, his son informed on September 6, 2018.