The rally comes a day after the Islamist-rooted Turkish government rejected a stern warning from the secular military over the disputed election, calling its interference unacceptable in a democracy.
The military's General Staff on April 27 said it is gravely concerned about the situation and determined to defend Turkish secularism.
The candidate for president, Foreign Minister Abdullah Gul, is a member of the ruling Islamist-rooted AK party. He has said today he will not withdraw from the race.
Secularist lawmakers boycotted the first round of parliament's vote for a new president, and have challenged its validity in the Constitutional Court.
(AP, AFP, Reuters)