The preliminary results of the legislative elections are expected to be announced by 8 p.m. local time.
Some 9.7 million people were eligible to vote in the country. However, the turnout is expected to be low as only 44 percent of the population cast their ballots by 4 p.m..
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.