In the five decades since Portugal threw off its fascist dictatorship, a presidential election has only once before - in 1986 - required a runoff, highlighting how fragmented the political landscape has become with the rise of the far right and voter disenchantment with mainstream parties.
The presidency is a largely ceremonial role in Portugal but wields some key powers, including in some circumstances to dissolve parliament, to call a snap parliamentary election, and to veto legislation.
Partial results with almost 80 per cent of the vote counted put Seguro at just over 30 per cent, while exit polls put him in the range of 30-35 per cent.
Ventura was at 25.6 per cent, above the upper band of his exit poll range of 19.9 per cent-24.1 per cent. Large cities such as Lisbon and Porto are counted towards the end.
Joao Cotrim de Figueiredo of the right-wing, pro-business Liberal Initiative party likely came third among a total of 11 contenders, according to the exit polls conducted for television channels RTP, SIC and TVI/CNN that placed him at 16.3 per cent-21 per cent.
The second-round vote is tentatively scheduled for February 8.
Last May, the anti-establishment, anti-immigration Chega, founded just about seven years ago, became the main opposition party in a parliamentary election, winning 22.8 per cent of the vote. As in much of Europe, the rise of the far right has swayed government policies, particularly on immigration, towards a more restrictive stance.
However, all recent opinion polls have shown Ventura losing the runoff due to his high rejection rate of more than 60 per cent of voters.
But Ventura sounded combative as he left a Catholic mass he had attended in downtown Lisbon.
"Now we need to unite the entire right wing ... I will fight day by day, minute by minute, second by second so that there won't be a Socialist president. We will win," he said.
with AP
