The snap election pits Catalan pro-independence parties against those who want the region to remain a semi-autonomous part of the country.
One exit poll suggested that separatist parties would win a majority.
It said the separatist Republican Left of Catalonia (ERC) and the unionist Citizens (Cs) were neck and neck in the race to become the biggest party in the new parliament.
Spain dismissed the separatist Catalan government in October after declaring the referendum to be illegal.
El Pais newspaper said on its front page that one million undecided voters could have the last word on December 21.
Polling stations opened at 09:00 local time and closed at 20:00, with the first official results expected shortly afterwards.
At 18:00 local time turnout was 68%, the Catalan authorities said – about 5% higher than the corresponding figure for the 2015 regional election.
Most results should have emerged by 22:00 local time.
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