Brazil is on tenterhooks as voters are deciding if the world's fourth-largest democracy should continue to be led by the far-right incumbent or whether to vote a left-wing former president back in.

Luiz Inácio Lula da Silva from the Workers' Party beat President Jair Bolsonaro from the Liberal Party by five percentage points in the first round, four weeks ago.

But the run-off could be close-fought.

The winner will be sworn in in January.

Polls opened at 08:00 local time (11:00 GMT) and are due to close at 17:00 (20:00).

President Bolsonaro cast his vote in Rio de Janeiro just minutes after the polls opened while Lula voted in his hometown of São Bernardo do Campo in São Paulo state.

Voting is electronic and results are expected within hours of the polls closing. The count can be followed live on the website of the electoral authority.

Voters from both sides of the political divide have endured an anxious four weeks since the first round.

Jair Bolsonaro won more votes than opinion polls had predicted but his main rival Lula fell short of the 50% of valid votes needed to win outright.