"The tight referendum result shows how deeply divided Turkish society is and that means a big responsibility for the Turkish leadership and for President Erdogan personally", Germany's Chancellor Angela Merkel and Foreign Minister Sigmar Gabriel said in a joint statement, calling for a "respectful dialogue" in Ankara with the opposition and all parts of Turkish society.
At a press conference Monday, Bulent Tezcan, deputy chairperson of the opposition Republican People's Party (CHP), said his organization would lodge an appeal with the Turkish Constitutional Court and the European Court of Human Rights for what is called breeches of voting laws.
Istanbul: A Turkish police helicopter with 12 people, including seven police officers, and a judge, crashed yesterday due to unfavourable weather in eastern Turkey.
Turkey's Supreme Electoral Council declared late Sunday that the "yes" camp in the constitutional reform referendum had won amid an outcry from the opposition that the voting was rigged.
Austria, which has repeatedly called for halting membership talks, called once more for them to stop.
But confidence has been dented by the worsening ties with Europe, mounting concerns about political freedom and civil rights after last year's failed coup, a resurgent conflict with Kurdish militants, and the threat from Islamic State in neighboring Syria and Iraq.
Erdogan's margin of victory in the referendum was razor-thin. Turkey's main opposition party urged the country's electoral board Monday to cancel the results of. He said the party would use all legal paths to challenge the vote. The change has raise concerns of "ballot-stuffing" - where extra votes are cast illegally to manipulate results.
"We will invite our founding chairman to our party and we will feel a huge elation to see him among us", he said.
His congratulations stands in stark contrast to the more cautious tone adopted by European leaders and a statement issued by the US State Department, which acknowledged the results but warned against further repression by the Turkish government of the political opposition.
Supporters of the "no" vote, (Hayir in Turkish) chant slogans during a protest against the referendum outcome, in Istanbul, Tuesday, April 18, 2017.
"Politically motivated comments against the YSK's decision to accept unstamped ballots are wrong", Celik said, referring to the electoral board by its Turkish acronym.
About 2,000 protesters gathered in Istanbul's Besiktas neighborhood on Wednesday evening over the referendum approving constitutional changes sought by President Recep Tayyip Erdogan.
The two leaders also discussed Turkey's support for the USA response to a Syrian chemical weapons attack and efforts to counter the Islamic State group, according to a White House summary of their phone call Monday. "We will follow closely how Turkey behaves on this".
There have been sporadic protests against the outcome in cities, including Istanbul, Ankara and Izmir.