ISTANBUL (Reuters) - Police in Istanbul have detained seven suspects, two of them believed to be members of Islamic State, who were plotting to carry out attacks in Turkey ahead of a referendum on Sunday, a police statement said on Friday.

The four Turks, two Syrian nationals and one Tajik aimed to “create chaos” in Turkey, police said. Two of the Turks were thought to have joined the ranks of Islamic State, which holds territory in neighbouring Syria and Iraq.

Islamic State has been blamed for at least half a dozen attacks on civilian targets in Turkey in recent months, including one on New Year’s Day at Istanbul’s Reina nightclub which killed 39 people.

The police said they had also seized identity cards, mobile phones and passports in the raids in four districts of Istanbul since last week.

Turks will vote on Sunday on constitutional changes that would give President Tayyip Erdogan sweeping new powers. Supporters say that will strengthen the country at a time when it faces threats from Islamic State and Kurdish militants. Opponents fear a lurch towards authoritarianism in Turkey.

Two opinion polls on Thursday showed a narrow majority of voters would vote in favour of the changes.

Security efforts have been tightened 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.

NATO member Turkey is part of the U.S.-led coalition fighting Islamic State and it launched an incursion into Syria in August to drive the jihadist group and Kurdish militia fighters away from its borders.