By TIM ARANGO

In years past the military elite would never have stood beside women wearing a symbol long at the center of Turkey’s struggle over the role of religion in public life. These men were heirs to the traditions of Turkey’s secularist founder, Mustafa Kemal Ataturk, who zealously banished religion from public life. They had for years refused to attend such gatherings — in protest of the headscarf.

For many Turks, the reception underscored an emphatic break from a past when civilian leaders were subservient to the military, and Islam was filtered from public life.

“The Turkish Army is now withdrawing from politics,” said Taha Akyol , a columnist for Hurriyet, a Turkish daily newspaper.

At a time when Turkey’s prosperity and its melding of democratic and Islamic values are being put forward as a model for an Arab world in turmoil, the country is facing its own internal power struggles — between Islamists and secularists, civilian leaders and military commanders. The outcome could not only determine the future of Turkey but, as it takes on a greater role in the affairs of the Middle East, also shape the region.

While many praise the diminished power of the military, critics say these struggles have also laid bare the deficiencies of Turkey’s democracy, pointing in particular to the Islamist-leaning government’s crackdown on dissent and the press — there are more journalists in jail here than anywhere else in the world. That has given rise to a chorus of frustration that was on vivid display in the streets Monday as Turkey celebrated its birthday.

In Ankara, the capital, thousands of secularist protesters clashed with the riot police after they went ahead with a rally to celebrate Republic Day, the holiday marking Turkey’s founding as a republic in 1923, that had been banned by the government of Prime Minister Recep Tayyip Erdogan, vaguely citing intelligence reports that the gathering could become violent.

“It is telling for the state of democracy when the right to celebrate the national holiday in one’s own peaceful way is strained,” wrote Yavuz Baydar, a columnist, in Monday’s edition of the daily newspaper Today’s Zaman.

Among the many changes brought about by the government of Mr. Erdogan, a pious Muslim whose rule has transformed Turkey’s economy but alienated the secular old guard, has been to decisively establish civilian control over a military that four times in the past 89 years has acted above the law to remove elected governments. In late September more than 300 military officers received prison sentences for conspiring to overthrow the government, in a trial known as the Sledgehammer case. The proceedings deeply polarized Turkish society, raised questions about the independence of the judiciary and seemed at times to rely on fabricated evidence. But the case represented a turning point in Turkish history by diminishing the power of the military, for decades the enforcers of secularism.

“The era of coups in this country will never return,” Mr. Erdogan said in a recent speech.

One news report, in anticipation of Monday evening’s reception, declared, “This symbolic act will mark the beginning of a new era in civilian-military relations in Turkey.”

The symbolism of the reception, as well as the Republic Day rallies in Ankara and Istanbul to protest what many secularists view as the increasing authoritarianism of Mr. Erdogan, underscored Turkey’s deep divides and the threat they see to secularism. Mr. Erdogan’s Justice and Development Party has roots in political Islam and close connections with Egypt’s Muslim Brotherhood.

In 2007 Turkey’s military sought to halt Abdullah Gul’s rise to the presidency because his wife, like Mr. Erdogan’s, wears a headscarf. And initially Turkey’s first lady, Hayrunnisa Gul, avoided attending certain public events in deference to the military’s sensitivities. At the reception Monday night, Mr. Erdogan alluded to that past by saying, in remarks reported by the NTV television network, “Shame on the people who did not let me in here with Mrs. Emine Erdogan until today.”

Turkey seems increasingly caught between its secularist past and an unknown future. It is undergoing a wrenching process of writing a new constitution to replace the one that was imposed by the military after a coup in 1980, which could result in a new system that enlarges the powers of the presidency, now mostly a ceremonial post. Mr. Erdogan plans to run for president in two years.

On Monday, several hundred people waving flags bearing Ataturk’s picture gathered on Istiklal Street, a pedestrian thoroughfare lined with shops and cafes that is the center of Istanbul’s vibrant nightlife and where the few covered women are more likely to wear Burberry headscarves than the full face veils common in places like Saudi Arabia.

“Turkey is secular and will remain secular!” was one chant.

“We are the soldiers for Mustafa Kemal!” was another.

Nilgun Tekir, a nurse, joined the rally with her husband and 4-year-old son, whom she pushed in a stroller. “We don’t want a fundamentalist regime like in Iran,” she said.

Murat Kucuk, 30, a restaurant owner, wore a black T-shirt bearing Ataturk’s visage while walking in the procession. “This is a counterrevolutionm” Mr. Kucuk said. “Today is Turkey’s biggest day. It’s our heritage from Mustafa Kemal.”

These tensions are more often displayed among the urban elite in places like Istanbul than among the more conservative masses of the Anatolian heartland where Mr. Erdogan draws much of his support. Such public displays can appear in unlikely places, as they did Sunday night after Serena Williams defeated Maria Sharapova in a tennis match here. During the award ceremony, politician after politician was booed loudly, even during a speech by one of Mr. Erdogan’s ministers, Fatma Sahin, promising to bring the 2020 Summer Olympics to Istanbul.

Afterward, on her Twitter account, Ms. Sahin wrote, “I invite those who do not understand the effort shown here, do not see the beauty in this championship, to grasp the place Turkey has reached. It is their duty to their country to appreciate what has been done here.”