Turkey has detained some 50 people suspected of being linked to the Islamic State (IS) militant group.
Turkish media said police had conducted predawn raids on 17 locations in Istanbul.
Anadolu Agency said those detained on October 18 were all foreign nationals and planned to travel to conflict zones in Syria and Iraq.
It did not provide information on their nationalities but said they had arrived in Turkey recently and were being monitored.
Last week, two suicide bombings targeting a peace rally in the capital, Ankara, killed 102 people.
Unconfirmed reports say the attacks were carried out by a Turkish cell of IS.
Under pressure from its allies, Turkey has been tightening security at its 910-kilometer-long border with Syria to prevent IS militants crossing in and out of Syria.