Some demonstrators also denounced a recent government probe into what authorities have said is an alleged plot to overthrow the ruling Justice and Development (AK) Party.
Turkish authorities have arrested nearly 200 people as part of the investigation into the ultra-nationalist Ergenekon group.
Prosecutors accuse the group of planning a campaign of bombings and attacks to undermine the Islamic-rooted government.
Government critics say the investigation into Ergenekon is the government's response to a failed 2008 lawsuit to ban the AK Party on the grounds of anti-secular activities.
Secularists accuse the AK Party of seeking to replace Turkey's secular system with one based on sharia, or Islamic law.
The AK Party rejects the allegations.