Recep Tayyip Erdogan says he has been returned as president of Turkey, according to unofficial results.
People have been voting in both presidential and parliamentary elections.
The ballots will transform the country from a parliamentary democracy into an executive presidency – something approved in a referendum last year.
Under the new system, the president will be able to form the government, appoint ministers, vice presidents and top bureaucrats, issue decrees, prepare the budget and impose states of emergency.
Opposition politicians had vowed to return Turkey to parliamentary democracy with strong checks and balances.
“The nation has entrusted to me the responsibility of the presidency and the executive duty,” 64-year-old Mr Erdogan said.
He also declared victory for the People’s Alliance, which comprises his AKP party and the smaller Nationalist Movement Party.
He said the alliance had a “parliamentary majority” in a new 600-member assembly, containing 50 more members than before.
Earlier, Mr Erdogan’s main rival for the presidency, 54-year-old former physics teacher Muharrem Ince, accused the country’s state-run Anadolu news agency of “manipulation”.
After the agency said 85% of ballot boxes had been counted, Mr Ince claimed it was only 37%.
Election observers and party members should not “abandon the ballot boxes”, he said.
But government spokesman Bekir Bozdag alleged Ince was “threatening and targeting” the agency.
Mr Ince was backed by the main centre-left main opposition, the Republican People’s Party, or CHP.
Rallies he spoke at in Istanbul, Ankara and Izmir drew huge numbers.
Five candidates ran against Mr Erdogan in the presidential race.