President Tayyip Erdogan's ruling AK Party had 43.1 percent of the votes with 90.17 percent of votes counted in Turkey's parliamentary elections, CNN Turk and local broadcasters said on Sunday.
Muharrem Ince, the main opposition's presidential candidate, stood at 29.4 percent nationwide, television channels said.
A count of less than 10 percent for the parliamentary election also showed that Erdogan´s ruling Justice and Development Party (AKP) was well ahead and set for an overall majority.
Voting in Istanbul along with his son-in-law and Energy Minister Berat Albayrak, Erdogan said he expected turnout to be strong in an indication of "how mature democracy is in Turkey".
Opposition parties and NGOs have deployed up to half a million monitors at ballot boxes to ward against possible electoral fraud.
Erdogan needs over 50% to retain the presidency in the first round, but these are still early results and the outcome could yet change. Critics say it will further erode democracy in the North Atlantic Treaty Organisation member state and entrench one-man rule.
Turkey's President and leader of the Justice and Development Party (AKP) Recep Tayyip Erdogan looks at his ballot before casting his vote at a polling station in Istanbul on June 24, 2018. Erdogan was expected to address supporters from a balcony of the headquarters building later in the evening.
"It's time for change", said Aynur, 40, an architect who cast her vote at a polling station near Istanbul's central Taksim Square.
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The voters had to present a medical report explaining their reason for being unable to come to polling stations. "With the presidential system, Turkey is seriously raising the bar, rising above the level of contemporary civilizations".
As of 15:50 GMT, Erdogan is leading in the polls with almost 60 percent of the vote after 20 percent of the ballot boxes were opened.
Although Erdogan dominated airtime on a pliant mainstream media, Ince finished his campaign with eye-catching mass rallies, including a mega meeting in Istanbul on Saturday attended by hundreds of thousands of people.
Mr. Erdogan blamed the coup on his former ally, US -based Muslim cleric Fethullah Gulen, and has waged a sweeping crackdown on his followers in Turkey, detaining some 160,000 people, according to the United Nations.
Turkey's voters will go to the polls Sunday for the country's second election since emergency rule was imposed after the 2016 attempted coup.
If Erdogan wins both the presidency and control of Parliament, observers worry that Turkey could continue a slide from authoritarianism to outright dictatorship.
The president's critics, including the European Union which Turkey still nominally aspires to join, say Erdogan has used the crackdown to stifle dissent. Both Erdogan and Ince have said they will lift the state of emergency as president. All we want is a fair competition.
However, if the pro-Kurdish People's Democratic Party (HDP) crosses the 10 per cent hurdle, the AKP alliance may lose its absolute majority in parliament.