The 10-day trip had been previously targeted by opponents throwing stones and eggs at the two-term former president, who despite his legal problems leads polls ahead of the October 7 election.
But the shooting incident late Tuesday sharply ramped up tension in the most turbulent leadership contest since Brazil's military dictatorship ended in 1985.
Lula and his Workers' Party said the shots were fired in an apparent ambush, with two bullets hitting one bus, one bullet hitting another, and the third bus, where Lula was traveling, escaping unscathed. No one was hurt.
Gleisi Hoffmann, leader of the Workers' Party, which Lula founded, said the incident should be investigated as a possible assassination attempt.
Center-right President Michel Temer expressed "regret" Wednesday and said "we need to reunite Brazilians. We need to pacify the country. This wave of violence, this climate of 'us against them,' cannot continue."
However, Lula's chief rival, right-wing former army captain Jair Bolsonaro, arrived in Curitiba for his own rally early Wednesday with a more combative message.
He called Lula "a scoundrel" and said "we can't accept elections without Lula being locked up."
Not mentioning the shooting incident, Bolsonaro instead repeated his campaign promise of loosening gun laws to arm citizens against criminals.
"I want a ... police that shoots to kill," he told several hundred cheering supporters outside Curitiba's airport, where he flew in before driving off to another campaign stop.
When Lula left office at the start of 2011, he was Brazil's most popular president on record, having presided over a commodities-fueled economic boom and winning plaudits for his social policies.
However, he also inspires passionate opposition and is blamed by the right and many in the center for Brazil's slide into the mammoth "Car Wash" corruption scandal that has shaken the country over the last four years.
Although dozens of other top politicians, including Temer, have also been charged or convicted, right-wing opponents see Lula as the graft scandal's biggest culprit. On the left, Lula is seen as the victim of politicized judges.
Illustrating the ugly pre-election atmosphere, center-right presidential candidate and Sao Paulo state governor Geraldo Alckmin initially reacted to reports of shots fired against Lula's buses by saying the leftists were "reaping what they sowed," Folha newspaper reported.
Today, Alckmin rowed back, tweeting that "all forms of violence must be condemned ... The country is tired of division and calls to conflict."
There was clearer condemnation of the incident from the economy minister and possible center-right presidential candidate Henrique Meirelles. He tweeted: "What happened yesterday ... was an attack on freedom of expression of a political leader and was inadmissible in a democracy."
Lula's biggest challenge right now is avoiding jail. On Monday, a court rejected his latest appeal against a 12-year-and-one-month prison sentence for taking a luxury apartment as a bribe.
That leaves him depending on a Supreme Court decision expected on April 4 if he is to remain free, let alone be allowed on the presidential ballot. Even then, he faces another six corruption cases.
Despite the daunting legal situation, Lula is a runaway favorite in opinion polls, with around 35 percent of voter intentions, followed by Bolsonaro with around 17 percent.
A veteran right-wing congressman, Bolsonaro is running as an anti-politician, often taking a leaf out of US President Donald Trump's book.
He has praised torture under Brazil's military dictatorship, insulted gays and even told a fellow politician she wasn't "worth raping." The more extreme Bolsonaro gets, the more it seems he thrills his supporters.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)