“Our nation has given me the mandate for the presidency,” Erdogan said in a victory speech in Istanbul, noting his declaration was based on unofficial results

Istanbul, Turkey (CNN)Turkish President Recep Tayyip Erdogan has declared himself the winner of a high-stakes election, surviving the most serious challenge yet to his political dominance and tightening his grip on the nation he has ruled for 15 years.

But the opposition cried foul, claiming that state media and the election commission had manipulated the results and saying it was too early to be sure of the outcome.

An official result is expected later Sunday. If Erdogan’s victory is officially confirmed, he will gain sweeping new powers when he resumes office. He narrowly won a referendum last year to transform the country’s parliamentary system to a powerful executive presidency, in what his critics called a blatant power grab.

Erdogan also said his ruling Justice and Development Party (AKP) and its allied Nationalist Movement Party (MHP) had together retained their control of parliament.

The results were a blow to Erdogan’s closest rival, Muharrem Ince and his Republican People’s Party (CHP), who had seemed on the verge of forcing the President into a damaging runoff and denying the AKP-MHP alliance a parliamentary majority.