President Tayyip Erdogan had 56.5 percent of the vote with half of the votes counted in Turkey's presidential election on Sunday, broadcasters said.
Muharrem Ince, the main opposition's presidential candidate, stood at 28.6 percent nationwide, television channels said.
Early results had been expected to give Erdogan and his Islamist-rooted AK Party a strong lead - one that could shorten as more votes are tallied across the nation of 81 million people.
If no candidate wins more than 50 percent in Sunday's vote, a second round run-off will be held on July 8.
The elections will complete Turkey's transition to a new executive presidential system, a move approved in a controversial referendum last year.
Erdogan, 64, is seeking re-election for a five-year term with hugely increased powers under the new system, which he insists will bring prosperity and stability to Turkey, especially after a failed coup attempt in 2016 that has left the country under a state of emergency. His ruling Justice and Development Party, or AKP, is hoping to retain its majority in parliament.
Still, Erdogan -- who has been in power since 2003 -- is facing a more robust, united opposition this time. Opposition candidates have vowed to return Turkey to a parliamentary democracy with strong checks and balances and have decried what they call Erdogan's "one-man rule."
Muharrem Ince, Erdogan's main challenger, warned civil servants involved in the vote count to do their jobs "abiding by the law" and without fear, suggesting they were under pressure by the government. He asked all Turks to be vigilant at polls and not be "demoralized" by what he called the possible manipulation of news.
Five candidates are running against Erdogan in the presidential race. Although Erdogan is seen as the front-runner, he must secure more than 50 percent of the vote Sunday for an outright win. If that does not happen, a runoff would be held July 8 between the leading two contenders.
"With these elections, Turkey is achieving a virtual democratic revolution," Erdogan told reporters after voting in Istanbul. He said turnout appeared to be high and that "no serious incidents" had occurred.