TURKISH President Recep Tayyip Erdoğan is taking on extensive new executive powers following his outright election victory in Sunday’s poll.
Parliament has been weakened and the post of prime minister abolished, as measures approved in a controversial referendum last year take effect.
Defeated opposition candidate Muharrem Ince said Turkey was now entering a dangerous period of “one-man rule”.
Erdoğan polled nearly 53% in the most fiercely fought election in years.
Ince received just 31%, despite a lively campaign attracting huge crowds.
Erdoğan, 64, has presided over a strong economy and built up a solid support base. But he has also polarised opinion, cracking down on opponents and putting some 160,000 people in jail.
In his victory speech on Monday morning, Mr Erdogan vowed to bring in the new presidential system “rapidly”.
The constitutional changes were endorsed in a tight referendum last year by 51% of voters. They include giving the president new powers to; directly appoint top officials, including ministers and vice-presidents, intervene in the country’s legal system and impose a state of emergency.