A last-ditch deal aimed at averting a split of the opposition vote will see CHP chief Kemal Kilicdaroglu name the popular mayors of Istanbul and Ankara as vice presidents should he end Erdogan's two-decade rule.

"We would have been eliminated had we split up," Kilicdaroglu told huge crowds of cheering supporters after emerging from hours of tense talks.

Erdogan faces the fight of his political life in what many view as Turkey's most consequential election since its birth as a post-Ottoman republic 100 years ago.

The 68-year-old leader needs to overcome the twin hurdles of an economic crisis and that aftermath of a devastating earthquake as he seeks to extend his Islamic style of rule until 2028.

Opinion polls point to a tight race that remains far too close to call.

But Erdogan's task had appeared to become a little easier when one of the main leaders of the six-party opposition alliance walked out of the talks on Friday.