Brazil stages its first presidential election debate Thursday with eight of the crowded field locking horns but also one notable absentee -- jailed frontrunner Luiz Inacio Lula da Silva.

With deeply unpopular President Michel Temer not seeking a new term and Latin America's biggest country in a deep funk after years of recession and corruption scandals, it is the least predictable election in decades.

The debate on TV Bandeirantes will not feature four of the candidates, who have too small a presence in Congress to qualify.

But the real missing piece in the puzzle will be former two-term president Lula, who leads in opinion polls but is serving a 12-year sentence for corruption and looks almost sure to be barred from the ballot.

A court rejected his request to take part in the debate by jail cell video link.

So far, TV Bandeirantes has not responded to requests from Lula's leftist Workers' Party to mark his absence with an empty chair at the debate, or to allow his vice presidential pick, former Sao Paulo mayor Fernando Haddad, to take his place.

But for the Workers' Party, even an absent Lula is a powerful weapon. With around 30 percent in the polls, he would lead a first round and easily win a runoff.

Because he has lost an appeal to his corruption conviction, he in theory will be barred under the Brazilian clean slate law.

But Lula's lawyers are pushing for an escape route in the courts.

The main candidates due to appear during the debate include right-winger Jair Bolsonaro, who is polling in second place after Lula, and his next biggest rivals: center-right former Sao Paulo governor Geraldo Alckmin and environmentalist Marina Silva.

Pessimism

The October polls will also see elections for 27 governors, all 513 congressional lower house deputies and two thirds of the 81 senators.

But so far voters appear hardly enticed by the prospect.

A poll published this month by the National Confederation of Industries showed 45 percent of Brazilians "pessimistic or very pessimistic" about the elections. While voting is obligatory, a third plan to cast spoiled ballots, the poll found.

Other polls point to between 33 and 41 percent of the electorate defying the law to skip voting.

"Unlike in other countries, there has been no new leader appearing in Brazil who is able to surf the wave of discontent," political analyst Matias Spektor told AFP. "Brazil's political system continues to generate a lot of frustration."

If Lula is barred, as expected, Bolsonaro from the conservative PSL party and Silva would be the main beneficiaries, according to current polling.

They stand out among the crowd with platforms "very focused on social discontent over socioeconomic factors and corruption and (in Bolsonaro's case) violent crime in the big cities," said Thiago Vidal, a political consultant.