President Barack Obama initially viewed Erdogan as a possible model for a new generation of Muslim leaders and traveled to Turkey early in his first year in office. Many worldwide observers decried the results, suggesting that the referendum had not taken place on a level playing field.
German Foreign Minister Sigmar Gabriel said that reintroducing capital punishment would be "synonymous with the end of the European dream".
Turkey's president Monday rebuffed a report critical of Turkey's handling of Sunday's historic referendum, saying Ankara would ignore its claims, APA reports quoting Anadolu Agency.
And a sharply worded report Monday by an worldwide monitoring group said the referendum "fell short" of full adherence to global standards.
This is "a damning verdict", says the BBC's Mark Lowen, and "is sure to embolden the opposition in its attempts to challenge the results". Election board officials have said that they were trying to avoid suppressing votes and that the decision was not unprecedented in Turkey's elections.
They also said a Turkish electoral board decision to allow as valid ballots that did not bear official stamps undermined important safeguards against fraud.
The president's role will now be dramatically expanded, changing from a mostly ceremonial role to a almost all-powerful position as head of government, head of state and head of the ruling party.
He made the call Monday after the "Yes" campaign won over 51 percent of the votes during Sunday's referendum, while the "No" campaign gained almost 49 percent.
Mr. Erdogan also flagged that he would take up the issue of reinstating the death penalty with Turkey's political leaders and may seek a referendum.
Meanwhile, global monitors with the Organization for Security and Cooperation in Europe (OSCE) raised concerns over a decision by the electoral board to accept ballots that did not have official stamps, as required by Turkish law, which critics said could amount to election fraud.