Demonstrators are fearful that Mr Erdogan will push through with an Islamist agenda if elected, an accusation he denies.
Police estimate that more than 300,000 people packed into Tandogan square today, holding placards of Mustafa Kemal Ataturk, the revered founder of the secular Turkish republic, and chanting "Turkey is secular and will remain secular forever".
The AK party has previously attempted to remove the country's ban on Islamic headscarves, as well as put religious leaders into positions of political responsibility.
Its members will decide by April 18th whether to put forward Mr Erdogan as their candidate, with its ruling majority likely to result in a simple seven-year appointment for the prime minister.
Mostly Muslim Turkey is currently attempting to join the EU.
Click here to run an energy price comparison, and see if you could be paying less for your gas and electricity.