Polls show Erdogan trailing the main opposition candidate Kemal Kilicdaroglu a day ahead of one of the most consequential elections in Turkey's modern history. However, if neither of them win more than 50 per cent of the vote and secure an outright win, the vote will go to a runoff on May 28.
Voters will also elect a new parliament, likely a tight race between the People's Alliance comprising Erdogan's conservative Islamist-rooted AK Party (AKP) and the nationalist MHP and others, and Kilicdaroglu's Nation Alliance formed of six opposition parties, including his secularist Republican People's Party (CHP), established by Turkey's founder Mustafa Kemal Ataturk.
Polls will open at 8am and close at 5pm local time. By late on Sunday there could be a good indication of whether there will be a runoff vote for the presidency.
Erdogan's campaign over the past month has focused on his government's achievements in the defence industry and infrastructure projects, and his assertion that the opposition would roll back such developments.
One of his talking points has been that the opposition is receiving orders from the West, and that they will bow down to Western nations' wishes if elected.
At a rally in Istanbul's Umraniye district, Erdogan recalled comments made by Biden and published by the New York Times in January 2020, when he was campaigning for the White House. At that time, Biden said Washington should encourage Erdogan's opponents to defeat him electorally, stressing he should not be ousted in a coup.
The comments, which resurfaced later that year in a video that made Biden the most popular topic on Twitter in Turkey, were condemned by Ankara at the time as "interventionist."
"Biden gave the order to topple Erdogan, I know this. All my people know this," said Erdogan, 69. "If that is the case, then the ballots tomorrow will give a response to Biden too," he added.
A US State Department spokesperson said Turkey was a long-standing US ally and Washington would be following the election closely, but added: "The United States does not take sides in elections.
"Our only interest is in the democratic process, which should be both free and fair. We trust that Turkish authorities will carry out the election in keeping with its long, proud democratic tradition and its laws," the spokesperson said.
Erdogan also criticised Kilicdaroglu for his comments on Russia, calling Moscow an important partner for Turkey. "Russia has been one of our most important allies regarding agriculture products," he said.
Turkey's Western allies have been irked by closer ties between Ankara and Moscow under Erdogan. Turkey is a member of NATO, which has stood staunchly behind Kyiv since Moscow launched a full-scale invasion of its neighbour last year but it has not imposed sanctions on Russia.
Kilicdaroglu told Reuters on Friday that his party has concrete evidence of Russia's responsibility for the release of "deep fake" online content ahead of Sunday's elections. He did not present the evidence and Reuters could not independently verify it.
But he added that if he wins the presidency he will maintain Ankara's good ties with Moscow.
Russia categorically rejects Kilicdaroglu's accusations of election interference, domestic news agencies cited Kremlin spokesman Dmitry Peskov as saying on Saturday.
"We are extremely disappointed with this statement of the opposition," Peskov said, adding Kilicdaroglu would not be able to provide proof of the supposed interference "because it does not actually exist".
