Hungary's parliamentary election on Sunday could produce the highest turnout on record, ruling Fidesz party parliamentary group leader Gergely Gulyas said, adding that it would give strong legitimacy to the next parliament.
Orbán has leaned farther to the right in recent years, becoming one of Europe's most prominent populist voices.
'Today will decide whether Hungary becomes an emigrant country or not - and I wouldn't like Hungary to be an emigrant country, ' Mr Vona said.
Casting himself as a defender of national sovereignty and "Christian Europe" against the "globalist elite", Mr Orban has spearheaded divisions between the European Union's western nations and eastern members such as Hungary and Poland.
Fidesz is seeking a third consecutive term in office and has won 49.15 percent of votes, with more than 64 percent of votes counted, according to the NVI, which said turnout had reached 68.80 percent.
"As long as the opposition is in a fragmented state. this migrant/refugee campaign is sufficient to keep (Orban's) voting base united, to keep it mobilized", said Balazs Bocskei, political analyst at the Idea Institute, a Budapest think-tank.
His main challengers are Gergely Karacsony, the candidate of the Socialist and Dialogue parties, and Gabor Vona from the nationalist Jobbik party.
Mr Orban has campaigned heavily on his unyielding anti-migration policies, though voters claim they are more concerned with poverty, government corruption and the country's underfunded health care system.
He said voter turnout would determine the outcome.
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The fierce anti-immigrant campaign has gone down well with around two million core voters of Fidesz.
Over 3.3 million voters had taken part by 1 p.m. (1100 GMT), for a turnout rate of 42.3 percent six hours before the end of voting.
Rebranding itself as a moderate "conservative people's party", its leader Gabor Vona has called for a change in government and railed against Mr Orban.
"We love our country and we are fighting for our country", he said.
If Orban wins again, he is expected to continue his economic policies, with income tax cuts and incentives to boost growth.
The PM and his wife Aniko Levai were pictured voting at a school in Budapest this morning, while Hungarian citizens in London were seen queuing in the rain to cast their ballots.
"Safety is first", said Julia Scharle, 27, holding her child outside the polling station where Orban cast his vote.
He says "the European Union is not in Brussels". The EU is now in Berlin, in Budapest, in Warsaw, in Prague and at Bucharest.
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.