Some 9.7 million people are eligible to vote in the country. The electorate is choosing between 13 political parties and three coalitions.
The elections are expected to result in a government led by one of the two main rivals, the right-wing Social Democratic Party (PSD), headed by incumbent Prime Minister Pedro Passos Coelho, and the left-wing Socialist Party (PS), led by Antonio Costa.
According to opinion polls, the coalition led by Coelho has the support of about 38 percent of voters, compared to 32 percent for the opposition Socialist Party, which indicates that the coalition could win between 99 and 114 seats in the 230-member parliament, just short of the 116 needed for an absolute majority. PS is likely to take between 78 and 95 seats.
At the same time, according to polls, up to 27 percent remain undecided, making a forecast difficult.
