More than 300,000 Turks marched today to try to stop the ruling AK Party from picking Prime Minister Tayyip Erdogan as their presidential candidate next week because of his Islamist roots.
The AK Party has its background in political Islam, and a possible presidency headed by Mr Erdogan has split this secular but predominantly Muslim country seeking European Union membership.
"Turkey is secular and will remain secular," shouted protesters as they waved national flags and banners of Mustafa Kemal Ataturk, revered founder of the republic which separated religion and state.
Tens of thousands of people were bussed into the capital Ankara from across the country to attend the rally in Tandogan square, one of the biggest gatherings in recent years.
A police official told Reuters the figure of more than 300,000 protesters was being circulated on police radio.
Some 10,000 police were on duty, but crowds were calm. Many of the speeches attacked the United States and the EU, calling on Turks to defend their country - echoing a rise in nationalism over the past year.
The AK Party has a big enough majority in parliament to elect Erdogan, or anybody else it chooses, to the seven-year post as head of state.
Senior AK Party member Egemen Bagis said the party's executive would likely announce their decide around April 23rd. Parliament is due to vote in May.
Mr Erdogan denies any Islamist agenda and says he has broken with his past and is now a conservative democrat. He has not yet to confirm whether he wants the position.
Despite being Turkey's most popular and charismatic politician who has presided over strong economic growth, opinion polls suggest a majority of people in the country do not want him to become president.