"What is being disputed today is change (...) I ask Colombia to make a constructive change of much more capacity", he said in his speech after Sunday's election day.
The candidate took the opportunity to recall the irony that his rival in the second round, Rodolfo Hérnandez of the League of Anticorruption Rulers (center-right), has allegations of corruption against him.
"My opponent is imputed for corruption. It is not a process of lies; they are real indications," he said from the red room of the Tequendama Hotel in Bogota.
Petro took the opportunity to highlight the role of women in his campaign and especially his running mate, Francia Márquez.
In Sunday's vote, Petro obtained 40.33 percent of the votes, followed by Hernandez, who received 28.16 percent of the votes, according to the National Registrar's Office, with 99.84 percent of the tables counted.
Both will meet again at the polls in the second round on June 19, where the successor of current president Iván Duque will be elected.
The right in disarray bets on populist Hernandez
The presidential candidate of the coalition Team for Colombia (far-right), Federico Gutiérrez, announced on Sunday that he and his vice-presidential formula, Rodrigo Lara, will vote in the runoff for Rodolfo Hérnandez, of the League of Anti-Corruption Rulers (populist-right).
"I want to express publicly that we do not want to lose the country and that is why Rodrigo and I will vote for Rodolfo next June 19", said the former Medellín mayor, who has been a fierce opponent of the left-wing candidate, Gustavo Petro.
Gútierrez said that he will not seek to be part of an eventual government of Hernández.
"Neither Rodrigo nor I will be part of the government of Rodolfo Hernandez (...) Looking between the two options, the engineer's is the most sensible," he said.