Under the new system, Erdogan leads the state's executive branch and has the right to appoint and dismiss vice-presidents, a newly introduced position, as well as ministers, high-level officials and senior judges - without parliamentary approval.
"Turkey is leaving behind a system which cost the country politically, socially, economically", Erdogan told the crowd, comprised also of dozens of foreign leaders and dignitaries. "(The governor) can be re-appointed at the end of this period". For some, it is the coronation of a new Turkey.
Erdogan, 64, says a powerful executive presidency is vital to drive economic growth, ensure security after a failed 2016 military coup and safeguard Turkey from war across its southern border in Syria and Iraq.
Erdogan has now become both the head of state and government; and the prime minister's post is abolished.
But Erdogan faces immediate challenges posed by Ankara's faltering bid to join the European Union and tensions between the United States and its North Atlantic Treaty Organisation ally. The president will introduce his cabinet members later this week. Erdogan's supporters see the changes as a just reward for a leader who has put Islamic values at the core of public life, championed the pious working classes and overseen years of strong economic growth.
President Muhammadu Buhari on Friday evening spoke with his Turkish counterpart, Mr Recep Erdogan, via telephone, and congratulated him over his re-election.
Opponents say the move marks a lurch to authoritarianism, accusing Erdogan of eroding the secular institutions set up by modern Turkey´s founder, Mustafa Kemal Ataturk, and driving it further from Western values of democracy and free speech.
Erdogan then held a snap election that saw him win almost 53 percent of the vote. "In other words, Turkey will be an institutionalised autocracy". The prime ministry will not exist in the new system.
Russian Prime Minister Dmitry Medvedev was present, in a new sign of the warm ties between Ankara and Moscow, as was Venezuelan President Nicolas Maduro, regarded with disdain by Washington but an ally of Erdogan. Erdoĝan has repeatedly clashed with strategic allies such as the U.S. and the European Union in recent years over the war in Syria, Turkey's accession to the EU, human rights abuses by Ankara, Europe's failure to support Turkey during the coup attempt, and rising Islamophobia in Europe.
The new cabinet, due to be announced at 1800 GMT, is expected to have a different look, especially after Erdogan said the government would include non-AKP figures.
Erdogan will face immediate challenges posed by an imbalanced if fast-growing economy and foreign policy tensions between the West and Turkey, a North Atlantic Treaty Organisation member.
The decree did not state an alternative term length or re-appointment process for the central bank governor, or what new requirements for deputy governors would be.
Inflation surged last month above 15 percent, its highest level in more than a decade, despite interest rate hikes of 500 basis points by the central bank since April.