Ms Rousseff (centre) is expected to face either Mr Neves or Ms Silva in a run-off vote. Photo: BBC

Early on, incumbent Dilma Rousseff had been expected to win outright.

However, the death of one of the main candidates and the popularity of his replacement, environmentalist Marina Silva, means the vote is now likely go to a second round.

Ms Rousseff's other main rival is Aecio Neves, a centrist, pro-business figure.

Voting began at 0800 (11:00 GMT), and voters will also select a federal parliament and state governors.

Mr Neves was narrowly ahead of Ms Silva, according to several opinion polls published on Saturday, which all put Ms Rousseff in a commanding lead with about 45%.

Brazil's voting system is all electronic and is likely to deliver a result within an hour or two of polls closing at 22:00GMT.

On the eve of the vote, the Superior Electoral Court moved to dismissed reports that the system was vulnerable to hacking, saying it was "fraud-proof", despite frequent attempts to hack into the ballots.

More than 530,000 voting machines are being used across the country

During Ms Rousseff's tenure as president, unemployment has been lower than under any of her predecessors, at about 5%. The minimum wage has risen and the number of undernourished Brazilians has been falling.

But the past 18 months have been marred by recession and corruption scandals, and protests over poor public services and the World Cup costs.

Despite widespread dissatisfaction with Ms Rousseff's centre-left Workers' Party (PT), polls early in the campaign suggested she was well ahead of her leading rivals - who were then Mr Neves and Socialist Eduardo Campos.

But then Mr Campos died in a plane crash and his replacement, Ms Silva, emerged as a surprisingly strong contender.

Last year, thousands of Brazilians protested at what they said was excessive spending on the World Cup

However, the former minister and environmental campaigner has lost ground in recent days.

Most analysts expect no clear winner to emerge in the first round. This mean the top two candidates are likely to face each other in a second round on 26 October.

Voting is mandatory for those aged between 18 and 70.