Supporters of Recep Tayyip Erdogan react as they listen to him during a campaign rally in Istanbul on Saturday. AP

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, with his popularity hit by a cost-of living crisis.

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.

"We will encourage people who could not go (to the ballot box in the first round). Will we complete the work that we left unfinished on May 14, with an overwhelming majority, hopefully by making the gap even wider tomorrow?"

Erdogan also paid homage to his executed Islamic predecessor on Saturday in a last-gasp attempt to rally his conservative base on the eve of a historic runoff election.

Erdogan's visit to Istanbul's Adnan Menderes mausoleum took him back to the man he cited when he called early polls for May 14 in a bid to ease his way to an unprecedented third decade of rule. Menderes was tried and hanged one year after the military staged a coup in 1960 to put Turkey back on a more secular course.

Erdogan survived a putsch attempt against his own Islamic-rooted government in 2016. "The era of coups and juntas is over," the 69-year-old declared after laying a wreath at his mentor's tomb. "I once again call on you to go to the ballot boxes. Tomorrow is a special day for us all."

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% of the vote in the first round versus 44.9% for Kilicdaroglu.

Erdogan got a boost earlier this week when Sinan Ogan - a nationalist politician who came third with 5.2% - endorsed him.

Kilicdaroglu, who is chair of Turkey's biggest opposition party, the CHP, meanwhile secured the endorsement of the anti-immigrant Victory Party for the runoff.

Victory for Erdogan would entrench the rule of a leader who has transformed Turkey, reshaping the secular state founded 100 years ago to fit his pious vision while consolidating power in his hands in what critics see as a march to autocracy.

According to data by the high election board (YSK), turnout for the first round was more than 87% including diaspora votes.

Polls will open at 8am (0500 GMT) and close at 5pm (1400 GMT) for more than 60 million voters. Unofficial results are expected to emerge by late on Sunday.

Observers say Turkey's votes are free of meddling on election days — but unfair because the odds are stacked against the opposition in advance.

"These were competitive but still limited elections," the Organisation for Security and Cooperation in Europe (OSCE) election observer mission's chief Michael Georg Link said after the first round.

Reuters