The centre-right governing coalition has vowed to continue austerity policies in place since 2011, after it was forced to seek a eurozone bailout.
The Socialists and other left-wing groups have criticised the cuts.
President Anibal Cavaco Silva called for a high turnout on Sunday, saying Portugal was facing a crucial moment.
The BBC's Alison Roberts in Lisbon says years of austerity have left Portugal's electorate deeply split, and with an unusually large number of undecided voters.
Socialist leader Antonio Costa has said that only a vote for his party is a "useful vote" against the government.
The centre-right Social Democratic Party led by Prime Minister Pedro Passos Coelho, and its right-wing ally, the People's Party, have put up joint lists of candidates in all electoral districts.
Unemployment has been falling for two and a half years, but many regard the recovery as fragile.
Read more: Coalition hopes to survive austerity anger
12%
of the workforce is unemployed
20%
of people live below the poverty line
485,000 emigrated from Portugal between 2011 and 2014
125% debt to GDP - the second highest rate in the European Union
People in Portugal are due to vote in a parliamentary election that is widely seen as a referendum on four years of spending cuts and market reforms.