Erdogan was set to win 54.1 per cent of the vote, way ahead of his main opposition rival Ekmeleddin Ihsanoglu on 37.4 per cent, Turkish television channels said, in results based on a 60 per cent vote count.
The third contender, Kurdish candidate Selahattin Demirtas, was set for 8.5 per cent of the vote.
If the trend is continued, the result will mark a personal triumph for Erdogan, 60, who has promised to be a powerful president with a beefed-up mandate, in contrast to the ceremonial role fulfilled by his recent predecessors.
"Our people will make an important decision for Turkish democracy," said Erdogan as he cast his vote in Istanbul alongside his wife Emine and two daughters and two sons.
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Erdogan indicated that he planned to revamp the post to give the presidency greater executive powers, which could see Turkey shift towards a system more like that of France if his ruling Justice and Development Party (AKP) succeeds in changing the constitution.
If Erdogan serves two presidential terms, he will stay in office to 2024 and already appears to be planning to preside over celebrations in 2023 for 100th anniversary of the foundation of the modern Turkish state.
Erdogan's opponents accuse him of undermining the secular legacy of Turkey's founding father Mustafa Kemal Ataturk, who established a strict separation between religion and politics when he forged the new state from the ashes of the Ottoman Empire.
"A ballot paper with only one name does not represent the democracy, it does not suit Turkey," said Ihsanoglu, 70, as he cast his ballot in Istanbul.
Erdogan ran a lavish three-month campaign that swamped those of his rivals, his face glaring down at pedestrians in Istanbul from gigantic billboards at almost every street corner.