Thousands of nationalist Turks marched in the capital, vowing to defend the secular regime against radical Islamic influences and urging the government not to make too many concessions in order to gain EU membership.
Some 12,000 people from more than 100 pro-secular associations waved Turkish flags on Saturday as they marched to the mausoleum of Mustafa Kemal Ataturk, the founder of modern Turkey.
Turkey is predominantly Muslim but is governed by strict secular laws that separate religion and state. Many fear that if left unchecked, Islamic fundamentalism will lead to a theocracy like that in Iran.
Retired General Sener Eruygur, president of the Ataturk Thought Association and former commander of Turkey's paramilitary forces, warned against purported plans by Turkish Prime Minister Recep Tayyip Erdogan of the Islamic-rooted ruling party to run for president -- a largely ceremonial post, but a symbol of secularism in Turkey.
Staunchly secular Turkish President Ahmet Necdet Sezer will retire in May, and the parliament -- dominated by Erdogan's legislators -- will choose the new president.
Since taking power in 2002, Erdogan has stoked secularist concerns by speaking out against restrictions on wearing Islamic-style headscarfs in government offices and schools and supporting religious schools. He also tried to criminalize adultery before being forced to back down under intense EU pressure, and some party-run municipalities have taken steps to ban alcohol.
A former Constitutional Court judge, Sezer has vetoed a record number of laws he ruled violated the secular Constitution and has blocked government efforts to appoint hundreds of reportedly Islamic-oriented candidates to important civil service positions.
"The gains of the republic were being rolled back one by one," Senal Sarihan, president of the Republican Women's Association, said in a speech during Saturday's rally. "Today is the day to rise up for the Republic."
Erdogan's government denies it has an Islamic agenda. It has also shown a commitment to joining the EU by enacting sweeping reforms that allowed the country to start membership talks last year -- a move greatly welcomed by the US.
Protesters rallied on Saturday, however, against EU demands to grant greater rights to Kurds and other minorities. Turkey is fighting a separatist Kurdish guerrilla group in a war that has killed more than 37,000 people since 1984, and many Turks regard granting more rights to Kurds as a concession to the rebels.