Former president Dilma Rousseff said Friday Brazil will be ungovernable if a court decision due next week blocks Luiz Inacio Lula de Silva from running in October polls, and upholds his corruption conviction.

"Any government that assumes power by winning the 2018 elections, without a transparent and correct electoral process, without maneuvers to invalidate candidates -- as in Lula's case -- will not be able to govern this country," Rousseff told AFP in an interview.

Lula was sentenced in July to 9.5 years behind bars after being convicted of corruption in Brazil's huge "Car Wash" graft scandal. The court in Porto Alegre said it will rule on his appeal on January 24.

That could decide whether Lula -- hugely popular during his 2001-2010 two-term presidency -- can take part in October 2018 presidential elections in which he is currently the frontrunner.

Speaking by phone from her home in Porto Alegre, Rousseff -- who was impeached in 2016 for breaking budget rules -- said she believed Lula can calm the South American country's stormy political waters.

"I don't think that insisting on the political use of the judicial system will stabilize and grow the country," she said.

"Lula can help turn the page, help in a transition period between now and a reconstruction.

"We are going to have to try to heal the wounds and mend the country."

After years of economic decline and corruption scandals, Brazilians are so far turning away from centrist, traditional candidates, with Lula a comfortable leader in the polls, despite his many legal problems.

Rousseff, 70, still believes that her impeachment was a parliamentary coup intended to cut a cycle of 13 years of leftist rule, and that the legal process against her mentor Lula is aimed at "dismantling" his socialist policies.

Rousseff lives near her daughter and grandchildren in Porto Alegre, where the court verdict on Lula's future will be delivered on Wednesday.

- Tensions ahead of verdict -

Lula's Workers' Party (PT) expects to flood the southern Brazilian city with its supporters ahead of the ruling.

The local authorities have called on the army to provide extra security. PT chairman Senator Gleisi Hoffman said that if they wanted to arrest Lula they would have to "kill people."

Rousseff distanced herself Friday from such talk.

"What there is, is indignation, and indignation is a peaceful, democratic feeling. That indignation is an expression of an awareness of injustice and political persecution," she said.

Rousseff remains convinced of a plot by judicial, political and business sectors to sideline the PT.

They wanted to "destroy the PT, destroy their greatest leader, but they went wrong because all the polls show a growing intention to vote for Lula."

Rousseff -- who is active on social media, where she describes herself as "president-elect of Brazil" -- says she is still evaluating a possible run for Congress, but for now is focusing her efforts on defending Lula, who faces six other corruption cases.

She believes that the man responsible for removing Brazil from the UN hunger map will not go to prison.

"I don't believe in that possibility. They're not going to want to make him a bigger hero than he already is."