of Prime Minister Recep Tayyip Erdogan, and what they said was his agenda to move Turkey away from the country’s secular legacy.

In a growing political showdown in Turkey, the country’s long-ruling secular establishment, backed by its powerful military, is confronting a new class of Islamic-influenced political modernizers, led by Mr. Erdogan. The confrontation has burst into public view over Mr. Erdogan’s choice for president: Abdullah Gul, his foreign minister and a close ally.

The presidency is the most important post in the secular establishment, and the prospect that it could be occupied by a man whose background is in political Islam is seen as deeply threatening. Voters do not choose the president directly; instead, the country’s parliament elects the president.

On Friday, the military, which has ousted four elected governments since 1960, warned that it would intervene again if the government did not demonstrate sufficient respect for the secular state.

The confrontation seemed to harden further today as Mr. Gul, an affable figure whose wife wears an Islamic headscarf, a practice that secular Turks find unacceptable for a presidential candidate, declared that he would not withdraw his candidacy.

“The process has begun and will continue,” Mr. Gul said today in Ankara, Reuters reported. “There can be no question of my candidacy being withdrawn.”

That stance has set his party, and the emerging middle class of religiously observant Turks that it represents, on a collision course with the secular establishment and the military.

On Tuesday, the constitutional court is expected to rule on whether Mr. Gul can indeed be a candidate. If it rules against Mr. Gul, Mr. Erdogan has promised to call national parliamentary elections, a move that may redraw Turkey’s political map, perhaps even more favorably for Mr. Erdogan and his party, the leading force in the current parliament.

Protestors joining the huge crowds in Istanbul wore and waved Turkish flags and chanted “Government resign!” in Caglayan Square, on the European side of this vast port city. Municipal authorities refused to give any official estimates of the crowd’s size: Both sides in the political standoff are trying to put them to political use. Aerial views showed a sea of Turkish flags, and crowds overflowing highway dividers.

The gathering seemed to draw Turks from a variety of backgrounds. The uniting factor seemed to be their distrust of Mr. Erdogan’s government, though they disagreed broadly on the reasons for distrusting him.

“Their constitution is the Koran,” said Yalcin Turkdogan, a 61-year-old architect who had not been to a protest since 1977.

The evidence, he said, was “their behavior, their speech, their ideas, and their religious education.”

For others, the sorest points were Mr. Erdogan’s policy of selling off state assets. His government has pushed to modernize the state, including sales of state-owned companies, a process that has made some Turks uncomfortable.

A serious problem for secularist Turks is the lack of an agile, articulate political party that could unite them and mount a serious challenge to Mr. Erdogan.

There appeared to be broad agreement that Deniz Baykal, the current leader of the main opposition party, was not up to the task.

Gokay Gedik, a 20-year-old student at Marmara University who had come to the rally with his friends, all members of the same rock band, described that political party with an idiomatic Turkish phrase that his friend said would translate roughly as “Blah, blah, blah.”