Venezuela’s President Nicolas Maduro says he is seeking an international arrest warrant for the former attorney general, Luisa Ortega Diaz, who fled the country last week.
Mr Maduro said Mrs Ortega and her husband had been involved in serious crimes.
The couple face court action in Venezuela which they say is politically motivated.
Colombian media reported they are expected to seek asylum in the US.
Mrs Ortega is Venezuela’s most vocal high-profile opponent of the Maduro government.
She is a former loyalist and was for years a committed member of the governing (USPV) party.
She and her husband fled first to the Carribean island of Aruba off the coast of Venezuela and from there flew to Colombia in a private jet.
In Colombia, Preisdent Juan Manuel Santos said his country would protect her and offer her asylum if she requested it.
The local media there speculated that she would have valuable information about Venezuela and Cuba’s relationship with Colombia’s former left-wing Farc rebels.
But on Tuesday Colombia’s migration authorities said she was heading for Brazil.
Ms Ortega issued a written statement saying she would be participating in a meeting in Brazil of prosecutors and attorneys from the regional economic group, Mercosur.
She said during the meeting she would make a statement on “the breaking of the constitutional thread in my country, Venezuela”.
She also said, “this event will allow me to show the world proof that will incriminate President Nicolas Maduro and those around him on serious charges of corruption”.
Luisa Ortega was sacked at the beginning of August on charges of “immoral acts” by Venezuela’s new National Constituent Assembly (ANC).
The creation of the assembly, which is packed with government supporters, was questioned by Mrs Ortega who said it was unconstitutional and would undermine the country’s elected national congress.
The National Assembly has also issued an arrest warrant for her husband, a former government supporter and left-wing congressman, Germán Ferrer.
He is accused of taking part in illegal activities within the attorney general’s office.
Other Venezuelan opposition members have also fled and are seeking asylum.
Chile said on Tuesday that it has granted diplomatic asylum to five Venezuelans who took refuge in its ambassador’s residence in Caracas.
The group includes four judges and an opposition politician.
The ANC has signalled its intention to go after opponents it sees as responsible for months of street protests against President Maduro.
He has defended the ANC saying it is designed to bring peace to the country.
The anti-government protests, in which more than 100 people died, were largely fuelled by opposition demands for early elections and the removal of Mr Maduro from office.
They took place against a background of the country’s collapsing economy with shortages of food and medicines and skyrocketing inflation and street crime.
Mr Maduro says the country’s dire political and economic crisis is being driven by a right-wing elite backed by the United States preparing the way for military intervention in order to take over Venezuela’s large oil reserves.
In press conference with international media, he vowed to defend Venezuela from US imperialism.