A sticker depicting former Brazilian President Luiz Inacio Lula da Silva that reads: "Free Lula" is seen in Rio de Janeiro. Picture: Sergio Moraes/Reuters

Rio de Janeiro  - Former Brazilian president Luiz Inacio Lula da Silva is topping opinion polls ahead of elections in October although he is behind bars, serving time on a conviction for corruption.

Lula has 37 per cent approval in opinion polls ahead of populist right-winger Jair Bolsonaro, who is polling at 18 per cent, according to figures released Monday.

Brazil's Workers' Party (PT) registered Lula last week despite him serving a jail sentence, the party announced.

Lula, 72, was found guilty of corruption and money-laundering in connection with the renovation of a beachside penthouse, which was bankrolled by a company seeking contracts with the state oil giant Petrobras.

He is serving a 12-year jail sentence in Curitiba in southern Brazil.

His party regards the judicial proceedings as a manoeuvre by the political right to remove leftist Lula from the presidential race.

The Supreme Electoral Tribunal (TSE) is expected to cancel Lula's candidacy on the grounds that the law does not allow people with criminal records to run for president. The court needs to announce its decision by September 17.

The PT might then replace Lula with its candidate for his vice president, former Sao Paulo mayor Fernando Haddad.

The October 7 elections come at a time of political and economic crisis, with much of the political class tarnished by corruption scandals and the economy still recovering from the 2015-16 recession.

The election is expected to go into a second round on October 28.