Preliminary results are expected an hour or two after polls close, and analysts say more general trends will emerge only slowly over the course of Sunday evening.
Polls have consistently put Orbans right-wing Fidesz party at least 20 points ahead of its nearest challenger Jobbik, a far-right party that has been moving towards the centre. Jobbik in turn has been a few points ahead of the centre-left Socialists.
Fidesz is widely tipped to win a third term, but turnout figures will be closely watched for any clues of a possible upset.
At the last election in 2014, almost 62 percent of eligible voters took part.
Experts say that if the turnout were to increase by even just a few percentage points, it could jeopardise Fideszs prospects for a solid majority, as opposition voters have previously been thought less likely to turn out than Fideszs more motivated voters.
The poll will be keenly watched for its possible wider ramifications in Europe.
Orban has clashed with EU institutions over his moves to clamp down on civil society and his fiery anti-immigration rhetoric, but he has drawn plaudits from other nationalist politicians and those on the far-right who look to him as an inspiration.
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