Brazil's President Jair Bolsonaro is expected to face his biggest rival for the presidency, popular leftist Luiz Inacio Lula da Silva, on Sunday for a debate ahead of October elections, after days of uncertainty over whether they would participate.

"See you at Band (broadcaster Rede Bandeirantes) tomorrow," Lula, who was president of Brazil from 2003-2010, tweeted on Saturday.

Bolsonaro has not officially confirmed his participation, but is also expected to appear, according to campaign sources quoted by local media on Saturday.

"At one point I thought I shouldn't go, now I think I should... I think my strategy is going to work," the far-right leader said in an interview with Jovem Pan radio on Friday.

The debate is the first in the campaign calendar ahead of the October 2 elections. Organizers have also invited four other candidates, including former finance minister Ciro Gomes and Senator Simone Tebet.

Polls have put Lula in the lead as the race heats up, with one published by the Datafholha Institute earlier this month showing the leftist leader taking 47 percent of the vote compared to Bolsonaro's 32 percent.

In 2018, when Bolsonaro won the election, he participated in the first two presidential debates -- but was then stabbed during a campaign rally, and after undergoing surgery he did not return for later debates.

Neither Lula nor another former Brazilian president, Fernando Henrique Cardoso, participated in debates before the first round when they sought re-election in 2006 and 1998, respectively.

On the eve of the debate, Bolsonaro and Lula both released campaign ads mainly focusing on the economy.

Lula criticized inflation and the spread of hunger, which affects more than 33 million Brazilians.

Bolsonaro attributed inflation to the pandemic, the war in Ukraine and drought; while promising to maintain a welfare program which transfers money each month to 20 million families.