Turkish police have detained 445 people suspected of links to the Islamic State group, state media say.
Co-ordinated early morning raids were carried out in 18 provinces, Anadolu News Agency reported, quoting anonymous security sources.
Most of those held are foreigners, among them 60 people taken into custody in the capital, Ankara, it said.
IS claimed responsibility for the New Year shooting attack on an Istanbul nightclub which left 39 people dead.
Last month police arrested an Uzbek national, Abdulkadir Masharipov, suspected of mounting the assault on revellers at the Reina nightclub.
In Sunday’s operations, nine suspects who are alleged to have been planning a terrorist attack were held in the Aegean province of Izmir, Anadolu reported.
Many of the suspects were detained in provinces bordering Syria – 150 in southeastern Sanliurfa and 47 in the city of Gaziantep.
A total of 18 people – 14 of them foreigners, including 10 minors – were detained in and around Istanbul, after operations at 20 addresses in the city.
The Turkish military has been battling IS inside northern Syria while seeking to push back ethnic Kurds in the region too.
A fierce struggle for control of the town of al-Bab has been under way since December, four months after Turkey launched its incursion across the border.
The country has suffered a string of attacks blamed on IS in the past two years, but also been accused of failing to do enough to stop militants crossing its borders and establishing a presence in its own cities.