Worldwide observers from the Council of Europe and the Organisation for Security and Cooperation in Europe (OSCE) said the referendum campaign was conducted on an "unlevel playing field" and the vote count itself was marred by the late procedural changes that removed key safeguards.
Several Turkish opposition groups claimed irregularities during the voting process and the Organization for Security and Cooperation in Europe said that the referendum fell short of global standards.
Germany's comments were echoed in France, where President Francois Hollande said: "It's up to the Turks and them alone to decide on how they organize their political institutions, but the published results show that Turkish society is divided about the planned deep reforms".
These contravened OSCE commitments and Council of Europe standards "regarding freedom and equality in the campaign", she said.
Presenting their preliminary findings at a news briefing in Ankara, Tana de Zulueta, head of the monitoring mission, described a litany of shortcomings.
The main opposition party has said it will challenge the result on the basis of irregularities-including the use of unstamped ballot papers-but the head of the country's electoral body has said the result is valid.
President Recep Tayyip Erdogan claimed victory Sunday night after a majority of Turkish voters appeared to have granted the president sweeping new powers.
The deputy chairman of the Republican People's Party (CHP), Bulent Tezcan, urged the vote to be scrapped to avoid all suggestions of foul play.
The new system would dispense with the office of prime minister and centralise the entire executive bureaucracy under the president, giving Erdogan the direct power to appoint ministers.
It sets a limit of two five-year terms for presidents, and also allows the president to remain at the helm of a political party.
The margin of victory was narrow and opposition leaders have demanded that the result be nullified. "Both the unfair campaign and the substantive reforms that will now be implemented take Turkey away from the prospect of a political alliance with the European Union".
Members of the Nationalist Movement Party who didn't support the Yes vote say they had campaign venues cancelled and in some towns and villages the AK party put pressure on local officials like mayors to push the yes vote.
It also stressed the importance of the political talks between Turkey and the European Union regarding the implementation of the constitutional changes. He doesn't expect a different decision from the Board, the Constitutional Court or the European Court from Human Rights.
The changes would come into effect with the next general elections, scheduled for 2019.
"Tayyip Erdogan may have done more good than the other big players (of Turkish politics). but I think of Tayyip Erdogan as just the best of the worst".
Umut Serin, a 32-year-old marine engineer who was working at a polling station in central Istanbul told CNN: "The government tried to reflect it as if the "yes" votes were clearly ahead, from the very start".