The results for the regional election will show whether people will support Catalonia's separatist movement or tension with Spain will continue. More than 5.5 million Catalans are eligible to cast their ballots.
Catalan authorities said on Thursday voting turnout is up by more than 5 percentage points in a vote that will determine the future of the Spanish region's independence movement.
That means more than a third of eligible Catalans cast their ballots in the first four hours of the voting.
The regional election board said 68.3 percent of Catalans had voted by 6 pm (1700 GMT; 12 pm EST) compared to 63.12 percent in the last election in 2015.
Surveys in recent weeks had predicted record turnout numbers.
TRT World's Sarah Morice brings the latest from Barcelona, Spain.
The regional election is being closely watched in and outside Catalonia to see if support for a separatist push will grow, subside or remain for the time being feeding political tension with Spain.
Elections traditionally take place on a Sunday, but a working day was chosen this time by central authorities to avoid voting on Christmas Eve.
More than 5.5 million Catalans are eligible to vote in the election. Results are expected later on Thursday after polling stations close.
Former regional leaders vote
The pro-Spanish unity candidate leading some of the polls in Thursday's Catalan election has vowed to end the social division that she blames on separatists in the northeastern region.
"We are going to fight very hard for Catalonia to return to normalcy," Ines Arrimadas, who heads the ticket for the Ciutadans (Citizens) party, told reporters after casting her vote in Barcelona.
The candidate of the socialist PSC party, Miquel Iceta, also voted in the regional capital calling for a "change of direction" in Catalan politics and vowing "to work to make progress possible."
Left-republican ERC's Marta Rovira noted in remarks to reporters after voting in the northern town of Vic that her party's candidate is in jail.
Former vice president Oriol Junqueras faces rebellion and sedition charges as part of a wider probe of efforts to make Catalonia independent.
"We vote today with the hope that will be able to impose itself," Rovira said.
Puigdemont watches from Brussels
Ousted Catalan leader Carles Puigdemont says that an election back home is "extraordinarily important" for the region's return to normalcy after Spanish authorities seized control this fall.
Speaking to Catalan reporters in Brussels, where he is evading a Spanish arrest warrant, Puigdemont said that "from the results will come the formula to recover democracy" in Catalonia.
Puigdemont fled to Belgium after central authorities sacked him for pushing ahead with unilateral independence for the region. Other members of his former cabinet, including his no. 2, were jailed under preliminary sedition charges.
"It's not normal, an election that takes place with candidates in prison and candidates in exile," Puigdemont said, referring to his own situation as an imposed absence.
The separatist politician, who can't vote from Belgium, thanked in a tweet an 18 year-old woman who cast a ballot paper on his behalf in a town near Barcelona.