Turkish President Recep Tayyip Erdogan 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”.
Mr Erdogan polled nearly 53% in the most fiercely fought election in years.
Mr Ince received just 31%, despite a lively campaign attracting huge crowds.
Mr Erdogan, 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.
Congratulations have come in from around the world, though some Western leaders have been slow to react. Russian President Vladimir Putin talked of Mr Erdogan’s “great political authority and mass support”.
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.
Some critics argue that Turkey’s new system lacks checks and balances.
Mr Erdogan says his increased authority will empower him to address Turkey’s economic woes and defeat Kurdish rebels in the country’s south-east.
Mr Erdogan was prime minister for 11 years before becoming president in 2014. Under the new constitution, he could stand for a third term when his second finishes in 2023, meaning he could potentially hold power until 2028.