After a decade out of power, the Socialists surged to a general election victory Sunday over the ruling Social Democrats, who were plagued by corruption scandals and the loss of a popular leader.
With votes from all but 14 of 4,221 precincts counted, the Socialists, headed by Antonio Guterres, had 109 seats in the 230-seat Parliament, seven short of a majority.
The center-right Social Democrats of Fernando Nogueira were second with 83 seats.
Eight seats remained vacant due to election boycotts and because votes from Portuguese emigrants had yet to be tabulated. The final distribution was expected to be announced by the end of the month.
Socialist officials said they would not seek a coalition partner if they fail to clinch an outright majority, but would instead form a minority government.
Many Portuguese had expressed fears that the lack of a clear winner could pave the way for a return to political instability in Portugal.