Turkish authorities have arrested 21 suspected members of the Islamic State group, according to state media.
They were picked up early on Friday in a series of raids in Istanbul and the nearby town of Kocaeli, as well as locations near the Syrian border.
Three of those detained are foreigners who were allegedly planning to cross into Syria.
Turkey has been accused of not doing enough to stop the flow of foreign fighters across its long Syrian border.
The Turkish government rejects those accusations. President Recep Tayyip Erdogan said on Thursday that the country had deported 1,300 foreigners thought to have links to IS.
The Turkish citizens arrested on Friday are suspected of recruiting foreign nationals to join the militant group.
How IS helps foreigners reach Syria
Analysis: Guney Yildiz, BBC News
Turkish security services have been preparing for this kind of sweeping operation since April 2014, a senior official told me. Unlike west European countries, Turkey was unable to carry out small-scale operations now and then, and wait to see how IS would respond, because IS is too close. So Turkey prefers larger operations when possible.
Foreign security services have also been helping, the official said. After a foreign fighter is deported from Turkey, the security services of the home country feed back intelligence from their interrogation – who this person has been meeting, and so on.
This enables Turkey to understand IS recruitment networks. The people they are targeting are Turkish nationals dealing with foreign recruits to IS.
The threat of IS attacks is constant. Previously, when IS was holding hostage Turkish officials seized in Iraq, Turkey was constrained, the official said. But now IS has no Turkish hostages so it is easier for Turkey to go ahead.