Voting has been halted at some Catalonia polling stations due to an internet outage, as rubber bullets were fired by riot police.
Catalan emergency services said 38 people were hurt, mostly with minor injuries, as a result of police action.
Spanish riot police moved in on a Barcelona street during disturbances over the banned Catalan independence referendum.
Meanwhile, electoral volunteers said they were unable to access census data because internet services to polling stations had been
Technicians were working to set up new domains for the website electoral managers needed to register polling data, said Jordi Sole, a volunteer assisting with the voting at the Collaso high school in Barcelona.
Guillem Castillo, an 18-year-old engineering student designated as an electoral official there, said technical problems halted the voting shortly after it opened.
Spanish media reported similar problems with internet in polling centres across Catalonia.
Voting was still going ahead at the polling stations lacking internet as people could still vote by paper.