Turkish President Tayyip Erdogan sought to build on his momentum going into Sunday's runoff presidential election, urging Turks to vote as the man aiming to defeat him called on electors to pull their country from "the dark pit" of his two-decade rule.

The Opposition has seen this month's presidential and parliamentary elections as their best chance yet of unseating Erdogan and unpicking far-reaching changes he has made to Turkey.

The two candidates are aiming to attract some 8 million voters who did not go to the polls in the first round.

A first round of voting on May 14 showed Erdogan with a lead over the Opposition's Kemal Kilicdaroglu, and Erdogan's Islamist-rooted AK Party and its allies secured a parliamentary majority in the initial vote.

Addressing flag-waving supporters in Istanbul's Beykoz district during his final campaign rally, Erdogan urged a strong turnout.

"Are we running to the polls tomorrow? Will we cast our votes from the early hours in the morning? We will not miss anyone that voted in the first round," he said, as the crowd chanted "yes" in excitement.

Dark pit

Kilicdaroglu, who is backed by a six-party Opposition alliance, said everyone who loves Turkey must vote.

"If you really want it, we'll all get out of this dark pit together," he wrote in a Tweet. "I am calling to all our people regardless of their view or lifestyle. This is the last exit. Everyone who loves their country should go to the ballot box!"

Erdogan won 49.5 per cent of the vote in the first round versus 44.9 per cent for Kilicdaroglu.