Polls agree on the triumph of Mr Orban's right-wing nationalist Fidesz party and its allied Christian Democrats, but a splintered opposition and Hungary's complex electoral system make the margin of victory hard to predict.
The autocratic Orban has campaigned heavily on his unyielding anti-migration policies, though voters say they are more concerned with poverty, growing government corruption and the country's underfunded health care system. Orban, Hungary's longest-serving post-communist premier, strongly opposes deeper integration of the bloc.
'Figures show that it will be an election with a high voter turnout. In central London, emigre Hungarians queued for hundreds of metres in the rain to vote, some waiting for more than two hours. Some pollsters have said voter turnout above 70 percent this time could cause the ruling Fidesz to lose its parliamentary majority.
A young woman queuing up to vote in Budapest said she is hoping for a breath of change after the election, but did not reveal who she was voting for.
Opposition parties have urged Hungarians to vote tactically for the opposition candidate with the best chance to defeat the Fidesz candidate in the 106 individual districts - but it is not clear how much impact that will have.
Orban has far-right admirers across Europe who like his tough line on migrants and a landslide win would show that his single-issue campaign, arguing that migration poses a security threat, had paid off.
The election is held every 4 years.
Turnout was higher than the final turnout of 61.73 percent in the last election in 2014 which gave Orban a landslide victory.
Orban accuses Soros and the organisations he funds of promoting mass Muslim and African immigration into Europe in order to undermine its Christian identity. It has curbed the powers of the constitutional court, increased control of the media and appointed loyalists to key positions.
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On Friday, at his closing campaign rally, he vowed to protect his nation from Muslim migrants, saying: "Migration is like rust that slowly but surely would consume Hungary".
Even if Fidesz does gain its expected parliamentary majority, analysts will be watching to see whether it falls short of the two-thirds "supermajority" that has enabled it to pass some of its most controversial bills.
"My little daughter must be my primary concern, to make her future safe".
The National Election Office initially said it did not intend to disclose any results while voters were still waiting outside polling stations.
Instead she said: "Droves of students have registered to vote in Budapest". The Socialists came in third with 14 percent.
However, one-third of voters remain undecided, and an upset is not being ruled out.
According to preliminary results with 85 percent of votes counted, National Election Office data projected Fidesz to win 133 seats, a two-thirds majority in the 199-seat parliament.
His business allies are also expected to expand their economic domains.
Rivals say that leaves open the possibility of an upset and are urging voters to turn out in record numbers.