Portugal's centre-right is leading a close race, according to exit polls, but will struggle to form a majority in parliament.
The Democratic Alliance aims to end eight years of rule by the centre-left Socialist Party.
It is expected to win 29-33% of the vote, ahead of the Socialist Party's 25-29%.
But the far-right party Chega (Enough) appears to be aiming to become a third force in Portuguese politics.
A key exit poll on public broadcaster RTP showed former soccer commentator Andre Ventura's party won between 14 and 17 percent of the vote after a campaign focused on corruption and immigration.
Five years after winning his first seat in parliament, Chega has set his sights on becoming kingmaker and the party hailed the historic night.
But Sunday's main winner appeared to be center-right leader Luiz Montenegro, whose supporters chanted “Portugal, Portugal” as his face appeared on TV screens as the exit poll results emerged. he shouted.
Four months after Socialist Prime Minister António Costa resigned over corruption allegations, although he was never named as a suspect, 10 million Portuguese people had the chance to vote in a snap election on Sunday.
Voter turnout was set to be the highest in years, despite the last election being held just two years ago.
The centre-right is not without its problems. The Social Democratic Party, which controls the Democratic Union, is involved in a regional scandal in Madeira.
Even if the Alliance wins Sunday's vote, it is expected to win around 91 seats in the 230-member parliament, far short of an outright majority.