If both the presidential and the parliamentary elections go against Erdogan, Turkey's politics, and possibly the country, could undergo significant change.
With 98 percent of votes counted, Erdogan secured 52.5 percent on Sunday, according to results released by state-run Anadolu Agency.
In a first for the nation, President Erdogan has won sole power of Turkey after beating leading opponent Muharram Ince to the top after a referendum past year saw Turkey vote in favour of scrapping their prime minister role.
Election observers and party members should not "abandon the ballot boxes", he said.
He added that the alliance led by the AKP had won the majority in parliament.
"Starting tomorrow, we will start working to realize the promises we made our people", Erdogan told flag-waving supporters in a victory speech delivered from the balcony of his ruling AK Party's headquarters in Ankara shortly after 3 a.m.
Voting already closed last week for Turkish citizens resident overseas, with just under 1.5 million out of just over 3 million registered voters casting their ballot, a turnout of just under 49 percent.
Erdogan has dominated Turkish politics for more than 15 years, first as prime minister and since 2014 as president.
"We urge the authorities to take action against the collective and open stuffing of ballot boxes", the pro-Kurdish Peoples' Democratic Party (HDP) said on Twitter.
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.
The CHP said it had recorded violations in particular in the southeastern province of Sanliurfa.
Also challenging Erdogan is 61-year-old former Interior Minister Meral Aksener, the only female presidential candidate in the race. Managing to get into parliament would be a significant for HDP, since nine of its lawmakers, including Demirtas, and thousands of party members were jailed during the campaign.
A woman votes at a polling station in a school during elections in Istanbul, Turkey, June 24, 2018.
"With the presidential system, Turkey is seriously raising the bar, rising above the level of contemporary civilisations". Erdogan called the election more than a year early in what analysts say was a pre-emptive move ahead of a possible economic downturn.
Opposition officials cried foul as soon as results began coming in, with the state-run Anadolu Agency announcing preliminary results more than two hours earlier than expected and opening with a massive lead for President Erdoğan and his party's alliance in parliament.
These elections were originally scheduled for November 2019 but were brought forward by Mr Erdogan.
Muharrem Ince, the main opposition's presidential candidate, stood at 29.4 percent nationwide, television channels said.
Turkey has been under emergency rule - which restricts some freedoms and allows the government to bypass parliament with decrees - for almost two years since an attempted coup in 2016.
Turkey held Sunday's elections under a state of emergency declared after a failed military coup in July 2016.