A sea of flag-waving demonstrators poured into the streets of the capital during the weekend to protest a possible presidential run by the pro-Islamic Prime Minister, whose party has been eroding secular Turks’ longtime grip on power.

With a crowd estimated at more than 300,000, the protest was one of the nation’s largest in decades. Red Turkish flags hung from balconies and windows and fluttered in the hands of protesters, who chanted, “We don’t want an imam as president!” and “Turkey is secular and will remain secular!”

Prime Minister Tayyip Erdogan has brandished his strong religious convictions, speaking out against restrictions on wearing Islamic-style head scarves in government offices and schools, and taking steps to bolster religious institutions in this country founded on the principle of secular rule.

He also tried to criminalize adultery before being forced to back down under intense pressure from the European Union, which Turkey is trying to join.