Some 300,000 people have demonstrated in Turkey's capital, Ankara, to demand that religion and politics should be kept separate in their country.
For many Turks, secularism is key to Turkey's identity
Protesters carried banners of Kemal Ataturk, the revered founder of the Turkish republic as a secular state.
The rally comes two days before the presidential election process begins and is intended to pressure current PM Recep Tayyip Erdogan not to stand.
His opponents accuse him of having an Islamic agenda - a charge he denies.
Tens of thousands of people were bussed into Ankara from across Turkey to attend the rally near Ataturk's mausoleum.
The area was packed with people, many of them draped in the red-and-white national flag and chanting anti-Islamic slogans.
"Turkey is secular and will remain secular forever," they shouted.
"I feel a little scared about the developments. I would not like to have an Islamic regime in Turkey," one demonstrator, who gave her name as Nursel, told the BBC.
"I would like to protect the secular system. That's why I am here. Especially as a female, this is very important for me."
For many Turks, maintaining the strict divide between religion and politics is key to keeping mainly Muslim Turkey a moderate, modern republic, says the BBC's Sarah Rainsford in Ankara.
Mr Erdogan says he will uphold Ataturk's values
There has been increasing speculation that Mr Erdogan's ruling Justice and Development Party, which has its roots in political Islam, will nominate him as its candidate to replace the current President, Ahmet Necdet Sezer, who steps down in May.
If nominated, he would be almost certain to be elected by parliament, where his party has a healthy majority.
But despite the large turnout at the rally, the secular establishment, including the army, has no real power to prevent Mr Erdogan from becoming president.
In five years in power, Mr Erdogan's government has overseen a wide range of democratic reforms.
However, our correspondent says that critics point to earlier attempts to criminalise adultery and appoint an Islamic central banker as signs of things to come.
Mr Sezer warned in a speech on Friday that the threat to Turkey of Islamic radicalism was stronger than ever.
Correspondents say Mr Sezer has used the post of president, although largely ceremonial, to speak up for secularism, vetoing laws he deemed in violation of the secular constitution.
Mr Sezer's warning came a day after the influential army chief said the country needed a committed secularist in the presidential palace.