Ecuadorean presidential candidate Fernando Villavicencio was killed on Wednesday, President Guillermo Lasso said, vowing his death will not go unpunished.
Local media had earlier reported Mr Villavicencio, a former legislator who had been polling at 7.5 per cent, had been shot at a campaign event in Quito.
“For his memory and his fight, I assure you that this crime will not remain unpunished,” Mr Lasso said on X, formerly known as Twitter.
“Organised crime has gone too far, but all the weight of the law will fall on them.”
Mr Lasso said he would host top security officials at an urgent meeting.
Videos on social media purportedly from the campaign event showed people taking cover and screaming as gunfire sounded.
Mr Villavicencio, from the Andean province of Chimborazo, was a former union member at state oil company Petroecuador and later a journalist who denounced alleged millions in oil contract losses.
He had made a report to the attorney general's office on Tuesday about an oil business, but no further details of his report were made public.
Mr Villavicencio was an outspoken critic of former president Rafael Correa and was sentenced to 18 months in prison for defamation over statements made against the former president.
He fled to Indigenous territory within Ecuador and later was given asylum in Peru.
As a legislator, Mr Villavicencio was criticised by opposition politicians for obstructing an impeachment process this year against Mr Lasso, which led the latter to call the early elections.
“Today more than ever, the need to act with a strong hand against crime is reiterated. May God have him in his glory,” fellow presidential hopeful Jan Topic said in a post on X.