National Assembly head Diosdado Cabello also alleged that Mr Nieves - who has left the country - had been bribed.
Mr Nieves was a prosecutor in the trial of opposition leader Leopoldo Lopez - a trial he now calls a sham.
He says Mr Lopez, who was sentenced to almost 14 years in jail, is innocent.
Mr Nieves left Venezuela last week for the United States and has been telling international media that Mr Cabello ordered the arrest of Mr Lopez to make sure the opposition leader was "taken out of the political game".
Human rights activists had long said that the trial against Mr Lopez was politically motivated and that it had not been conducted fairly.
Mr Nieves, one of the lead prosecutors in the case, has given details which apparently back up activists' concerns.
The former prosecutor alleges that evidence against Mr Lopez was fabricated and witnesses were told to give statements which would ensure the opposition leader's conviction.
Mr Nieves said he could no longer live with the guilt of having sent an innocent man to prison and that was why he had left for the US, where he is planning on seeking asylum.
Venezuela's attorney-general, Luisa Ortega, sacked Mr Nieves on Monday and said he had acted contrary to his oath of office.
Mr Cabello, whom Mr Nieves named as the main man behind Mr Lopez's alleged sham trial, said the former prosecutor had been bribed to make the claims.
"Franklin Nieves [is] a thug to whom they offered $850,000," he said, speaking on his weekly television show on Wednesday.
He also said that Mr Lopez had got off lightly with his sentence of 13 years and nine months in prison.
Mr Cabello argues that the opposition leader should not have stood trial for inciting violence but for murder.
He blames Mr Lopez for the deaths of 43 people who died in months of anti-government protests in early 2014.
The 43 came from both sides of the political divide, but Mr Cabello says Mr Lopez - through his speeches and messages on social media -  triggered the violence which led to their deaths.
Mr Lopez has always maintained his innocence and is currently appealing against his sentence.
The president of Venezuela's National Assembly has labelled Franklin Nieves - a former prosecutor who has said the judiciary convicted an innocent man - a "thug" and a "traitor".