Al Jazeera's Barnaby Phillps met many women at the rally who said they feared that their way of life, what they call a modern way of life, was under threat by the possible spread of "political Islam".
The rally came a day after Erdogan's government rejected a warning from the military over the country's disputed presidential election, calling its interference unacceptable.
Abdullah Gul, Turkey's foreign minister and election candidate for the ruling Justice and Development Party (AKP), said on Sunday he would not withdraw his candidacy after he failed to win a first-round victory in a parliamentary vote on Friday.
"We have seen this theatre many times... [there is] no way other than democracy"
Srimedya, Bursa, Turkey
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"It is out of the question for me to withdraw my candidacy in any way," he said.
Although Gul is the sole candidate in the election, he missed out on election in Friday's vote due to a boycott by opposition groups who object to Gul's alleged Islamist past.
The result of the vote prompted a military statement that some interpreted as a robust defence of Turkey's secular character.
The main opposition Republican People's Party has asked the constitutional court to cancel Friday's vote, arguing that the session of parliament was inquorate.
Founded in 1923 following the collapse of the Ottoman empire.
Army follows secular principles set out by Mustafa Kamal Ataturk, founder of modern Turkey.
Officers are also trained in dancing and etiquette.
It carried out two coups in 1960 and 1980.
It also managed to topple two other governments in 1971 and 1997 in "coups by communique".
Now it the second biggest army in Nato after the US.
If the court agrees, general elections set for November 4 could be brought forward.
If not, Gul could be president in a third round vote on May 9, when he needs only an absolute majority in the AKP-dominated parliament.
The prospect of Gul as president has alarmed secularists who suspect the AKP of having a secret Islamist agenda.
The ruling party argues that it is now committed to secular principles.
Sunday's protest comes after 300,000 pro-secular Turks demonstrated in Ankara two weeks ago, and highlights widening divisions between Turkey's secular and Islamic camps.