Some 9.6 m voters are to choose from among six candidates competing for the presidency, with first results expected shorlty after poll stations close at 1900 GMT.

Cavaco Silva, 71, an economist and member of the opposition Social Democratic Party, has touted his experience as the Mediterranean state remains mired in debt and struggling to avoid a bailout.

"Never has the situation in our country been so critical," Cavaco Silva said on the last day of campaigning on Friday. "This is not the moment for adventure. Others watching us need to see credibility."

Seeking a second term, Cavaco Silva has warned voters against engaging in a any "social experiment" at the polls as well as "the very high cost" of a second round if he fails to secure an outright win on Sunday.

Record abstention rates are expected, however.

Voters, worried by rising unemployment and poverty as Lisbon puts into effect its third austerity plan in a year, have shown apathy over the poll.

Local media said the start of voting Sunday was slow but blamed the low turnout on the cold weather.

Several villages had announced boycotts of the vote to back local demands, ranging from access to mobile phone networks or larger cemeteries.

Amidst little debate, the incumbent president has criticised the socialist-ruled government for failing to react quickly to shore up its dwindling public finances.

Polls show that Cavaco Silva, backed by the conservative minority in parliament, should be re-elected with a huge lead over his main rival, poet Manuel Alegre, 74, who has the backing of the governing socialists and the far left.

Alegre, however, is known for his differences with Prime Minister Jose Socrates and the Socialist Party has kept to the background during campaigning. A survey by the Marktest Institute forecast that 39 percent of the socialist electorate would cast their ballot for Cavaco Silva.

Throughout the campaign, Alegre has repeated he was "not the candidate of the government", but positioned himself as a "free man" carrying "new hope for Portugal" against "the dictatorship of the financial markets".

Though it carries great moral authority, the Portuguese presidency has limited executive power though the president can dissolve parliament.

Since the end of Portugal's dictatorship in 1974, all incumbent presidents standing for a second term have been re-elected.

In the last presidential poll in 2006, Cavaco Silva, who also served as prime minister from 1985-95, won on a first round against a fractious left, whose main contenders were Alegre and former Socialist president Mario Soares.

In the capital Lisbon, the campaign has been virtually invisible, with no posters on walls or leaflets handed out. A pre-poll survey last week forecast absention rates to be at least 35 per cent.

Portugal last week passed a key credibility test by raising 1.25 bn euros (1.70 bn dollars) for long-term debt at slightly reduced interest rates from a previous sale in November.

But analysts said it is only a matter of time before Lisbon seeks help as rates could still rise to levels that would require bailouts, in the same way that Greece in May and Ireland in November eventually caved in.