Tens of thousands Turks, waving national flags and chanting slogans, rallied in the capital Ankara on Saturday to voice their determinations to protect the secular republic.
Many participants, who arrived in Ankara from different parts of the country, started to march towards the city's Tandogan Square early in the morning.
They are let inside the demonstration area under tight security measures and after passing through certain check points.
Carrying a two-km-long Turkish flag, the participants shouted " Turkey is secular and will remain secular forever."
The rally came amid widespread concerns that Prime Minister Recep Tayyip Erdogan will run for president in May.
The country's secularists fear that if Erdogan becomes the next president of the secular Muslim country, his ruling Islamic-rooted Justice and Development Party (AKP) will be able to implement an Islamic agenda without opposition.
Turkey's parliament, which is dominated by lawmakers from Erdogan's party, will elect a successor next month to President Ahmet Necdet Sezer, a staunchly pro-secular president, whose seven- year-term will expire on May 16.
While delivering a speech at the War Academies in Istanbul on Friday, Sezer warned that Turkey's secular regime is facing an unprecedented threat, saying domestic and external forces are cooperating to undermine that regime.
His statement was considered as a warning targeted at Erdogan, whose party has not announced its candidate for the presidency.
Sezer also argued that foreign powers are trying to turn the secular republic into a democratic republic in an effort to transform Turkey into a "moderate Islamic republic."
"Societal developments in the past 15-20 years, religious messages, attacks, attitude in favor of compulsory headscarves for women and appointment of individuals, who are graduates of theological schools, to government posts, all clearly show the threat to the Turkish regime," he noted.