Erdogan's "yes" camp won the referendum with 51.4 percent of the vote against the "no" votes 48.6 percent, according to official final results announced last week.
Critics fear that the new presidential system approved in the referendum will lead to a one-man rule. But an amendment that reverses a requirement for the president to be non-partisan and cut ties with their party come into effect immediately, allowing President Recep Tayyip Erdogan to return to the Islamic-rooted Justice and Development Party, or AKP. Erdogan and his supporters argue that a strong presidency will bring stability and more efficient government.
He signed the paperwork to become a member to thunderous applause before a rendition of the national anthem, an AFP photographer said.
Erdogan arrived at the AKP headquarters in a 13-vehicle motorcade, the five-minute journey broadcast live on television channels.
Prime Minister Binali Yildirim said in his speech at Tuesday's ceremony that the president will be nominated to become chairman of the AK Party at an extraordinary congress on May 21.
"A chairman of a political party can not be a president of 80 million", Kilicdaroglu told a CHP party meeting in the parliament in Ankara.
Erdogan, who has four children, has described the AKP as his "fifth child" and has never made a secret of his desire to return to the fold. "If you don't open, then goodbye", Erdoğan said.
Palestinians say dozens wounded at protests for prisoners
There've been fresh clashes in the West Bank between Israeli forces and Palestinians protesting over prisoners in Israeli jails. About 6 500 Palestinians are held by Israel for offences ranging from stone throwing to deadly attacks.
"From now on there is no option other than opening the chapters you have not yet opened". If you open, then great.
President Recep Tayyip Erdogan says the European Union must open new chapters in Turkey's accession negotiations.
"Chapter" is the name for policy areas which are being addressed in the long-running talks on Turkish membership of the EU.
"First you have to handle these chapters and fulfill your promises". Then we will sit at the table and talk. "If not, we have nothing to discuss with you", he said.
The latest chapter was opened in June 2016 on financial and budgetary provisions while one chapter (science and research) has been provisionally closed.
The Turkish head of state has often railed against European leaders and threatened to hold a Brexit-like referendum on Turkey's membership talks.