Mr Villavicencio, 59, an ex-lawmaker and journalist with a track record of exposing corruption, was shot dead last week after leaving a campaign event in the capital Quito despite his own government-provided security detail.

His Build party, or Construye in Spanish, announced on social media they had tapped Andrea Gonzalez to replace the slain candidate in the 20 August vote.

Mr Villavicencio's widow criticised the party's replacement as unlawful and said she holds the state directly responsible for her husband's murder.

Veronica Sarauz told reporters: "The government still has to provide a lot of answers for everything that happened," she said, after arriving at the press conference with an armed police escort and wearing a bullet-proof vest and helmet.

Ms Sarauz described the party's decision to choose Ms Gonzalez as "arbitrary" and said it breaks a law that forbids the vice presidential candidate from stepping down.

The national electoral council must still approve the party's stand-in candidates.

Six suspects - all Colombian nationals whom police accuse of links to criminal groups - have been charged with the murder.

They remain in custody after a judge on Thursday ordered they remain behind bars as the criminal investigation continues.

Incumbent president Guillermo Lasso declared three days of national mourning and a state of emergency that involved extra military personnel deployed throughout the country following Mr Villavicencio's assassination.

Ms Gonzalez, an environmental activist who has not previously held public office, was selected by Mr Villavicencio to be his running made in the snap election called by Mr Lasso.

While ballots have already been printed, by law votes for Mr Villavicencio will automatically transfer to the party candidate.

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The South American nation of some 18 million people has seen a rising tide of violence in recent years, including a sharp increase in the murder rate.

Mr Villavicencio had been polling around the middle of the pack in a field of eight candidates prior to his assassination.

Beyond security, employment and migration have emerged as major campaign issues.

The South American country will vote in open primary elections on Sunday that will give an indication of how general elections in October will go.