ISTANBUL, Turkey — At least half a million Turks chanting anti-government slogans and brandishing the national flag gathered in Istanbul yesterday as tensions rose between the country ‘s Islamist prime minister and the fiercely secular army.

“Turkey is secular and will remain so,” roared the crowds as speakers took turns delivering fiery speeches extolling the virtues of Kemal Ataturk, the founder of modern Turkey.

“No to America, no to E.U., down with the government,” in two weeks, came after a dramatic warning late on Friday from the chief of general staff, detailing the dangers posed by radical Islam.

In an official statement, the general staff declared, “It should not be forgotten that the Turkish Armed Forces is one of the parties to this [secular versus Islamists] debate and is the absolute defender of secularism.”

The army added that it reserved the right to “take action” as warranted. The salvo sent shockwaves through the political establishment, with some analysts calling the generals’ words a threat to seize power. “No to a coup” declared the headline of the mainstream daily Sabah.

Western diplomats said the military’s declaration had dealt a further blow to Turkey’s efforts to join the European Union and could unravel more than four years of economic growth and political stability under the country’s mildly Islamist government led by Prime Minister Erdogan.

But these concerns were not evident at yesterday’s protest. “Secularism is more precious to us than democracy,” said Cenk Kutludemir, a university student. “And the E.U. won’t ever accept us anyway, to hell with E.U.”

The Turkish army, which views itself as the custodian of Ataturk’s legacy, has overthrown four governments since 1960. The generals’ persistent meddling in politics is seen as one of the foremost hurdles to Turkey’s efforts to join the European Union.

Some western diplomats say Mr. Erdogan’s efforts to reduce the army’s power lie at the root of his quarrel with the generals. The trigger for the latest row was the election by parliament of a new president to replace the incumbent, Ahmet Necdet Sezer, who will step down in May.

Mr. Erdogan, whose party commands a comfortable majority in the house, had been eyeing the job himself. But sustained secular opposition, culminating in a huge anti-government rally in Ankara last week, forced Mr. Erdogan to nominate his robustly pro-Western foreign minister, Abdullah Gul, for the top post.

But the military remains unswayed, not least because Mr. Gul’s wife wears the Islamic headscarf that women are banned from wearing in parliament, government offices, schools, and universities.

Should Mr. Gul be elected, his wife would be the first presidential spouse to cover her head, a vision that sends chills down secular spines.

“A woman who covers her head cannot sit in Ataturk’s palace,” fumed Nesrin Akkoc, one of thousands of women who traveled from the Western port city of Izmir, a bastion of secularism, to take part in yesterday’s rally.

“Turkey will not become another Iran,” she said.