Ten Turks and nine Syrian citizens connected to the Islamic State group (IS) were picked up during raids by police and intelligence officers in the city, state-run news agency Anadolu said.
The detentions came after Izmir police launched an investigation into planned attacks aimed at stopping people from going to vote, Anadolu said.
It added the suspects were planning to "sabotage" voting, without specifying how.
The 19 suspects were alleged to have been in contact with Is;amic State members inside Syria, Anadolu said.
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Turks will vote on Sunday to decide whether to approve constitutional changes that would create an executive presidency and axe the role of prime minister.
Turkey has been hit by a series of attacks in the last 18 months blamed on IS and Kurdish militants. The jihadist group has threatened further attacks.