Hungarians are going to the polls in large numbers, in a vote that could bring down Prime Minister Viktor Orbán after 16 years, and have significant repercussions for the rest of Europe, the US and Russia.
Most polls favour Péter Magyar, who formed a grassroots party, Tisza, after splitting from the ruling Fidesz party. After voting in Budapest, he said if he won he would bolster Hungary's position in the EU and Nato and move against corruption.
Orbán told reporters after casting his ballot "I am here to win" and, asked if he had underestimated his rival, said "I don't underestimate anyone".
Voting takes place until 19:00 (17:00 GMT) and results will start to come through during the evening.
After five hours, a record 37.98% % of the electorate had voted, a dramatic 12-point increase on four years ago and an indication that voters are far more mobilised this time around.
Orbán had turned tensions up a notch ahead of the vote, claiming the opposition would "stop at nothing to seize power", and Magyar responded by appealing to voters not to give in to "Fidesz pressure and blackmail".
After 16 years of Orbán running Hungary with what the European Parliament termed a "hybrid regime of electoral autocracy", Magyar and his Tisza party are promising "a change of regime", a reset of relations with the European Union and an end to close relations with Russia.
