A record 10 candidates are competing in Portuguese presidential elections with centre right politician and TV pundit Marcelo Rebelo de Sousa tipped to win.
While the post of president is mainly ceremonial, the head of state can dissolve parliament in a crisis.
A fragile coalition of left-wing parties currently governs Portugal after inconclusive elections in October.
The first results are expected at about 19:00 GMT, an hour after polls close.
Mr de Sousa, 67, is known as “Professor Marcelo” to his supporters and has been involved in politics since his youth, helping to establish the centre-right Social Democratic Party.
Polls suggest he will surpass the 50% mark required for an outright win in Sunday’s voting. If none of the 10 candidates passes this threshold, a run-off will be held on 14 February.
Mr de Sousa has the support of right wing parties but says he will not be reliant on them. He has pledged to do everything he can to ensure the current government’s stability.
If elected he will succeed Anibal Cavaco Silva, a conservative who served two consecutive five-year terms.
With analysts expecting a low turnout following a low-key campaign there is still a possibility that Mr de Sousa’s two left-of-centre rivals – former university dean Antonio Sampaio da Novoa and former Socialist health minister Maria de Belem Roseira – might upset the opinion polls.
Portugal opposition topples government
A centre-right coalition won the most votes in October’s election but lost its overall majority in November to be replaced by an alliance of left-wing parties which rejected its programme in parliament.
Portugal was one of the countries hardest hit by the crisis in the eurozone, accepting an international bailout in exchange for sweeping cuts.