Istanbul police announced that they have detained five suspected ISIS members, the state-run Anadolu news agency said on Friday.
Some were believed to be planning an attack in Turkey ahead of Sunday’s referendum, it added.
Anadolu said three of the detained people were suspected of planning an attack in the name of ISIS. Two others, including one of Tajik origin, had travelled to “conflict zones” and carried out operations for the jihadist group.
ISIS has been blamed for at least half a dozen attacks on civilian targets in Turkey in recent months, including a New Year’s Day attack on Istanbul’s Reina nightclub which killed 39 people.
ISIS has called for attacks against the referendum in its Arabic and Turkish publications this month, arguing that voting is opposing God and branding Muslim voters as “disbelievers.”
NATO member Turkey is part of the US-led coalition against ISIS and launched an incursion into Syria in August to drive the jihadist group and Kurdish militia fighters away from its borders.
Turks will vote on Sunday on changing the country’s political system and giving President Recep Tayyip Erdogan sweeping new powers. Two opinion polls on Thursday showed a narrow majority of voters would vote in favor of the changes.
Security efforts have been heightened ahead of the vote, but Kurdish militants on Wednesday claimed responsibility for a bomb attack on a police compound in southeast Turkey that killed three people.