The vote, whose outcome was welcomed by Ankara ally Saudi Arabia, has major implications for the long-standing EU bid of Turkey, also a key NATO member.
Narrowly won by President Recep Tayyip Erdogan, the referendum asked voters to boost the powers of the Turkish head of state -- a move that rights watchdogs have said could fatally weaken democracy in the linchpin country.
Here are some of the main reactions:
International monitors
"The referendum took place on an unlevel playing field and the two sides of the campaign did not have equal opportunities," said Cezar Florin Preda of the joint mission of the OSCE Office for Democratic Institutions and Human Rights (ODIHR) and the Parliamentary Assembly of the Council of Europe (PACE).
"The campaign rhetoric was tarnished by some officials equating 'No' sympathisers with terrorists," added ODIHR mission head Tana de Zulueta.
European Union
The EU noted "the close referendum result and the far-reaching implications of the constitutional amendments" and called on the Turkish authorities "to seek the broadest possible national consensus in their implementation."
The statement was issued by European Commission head Jean-Claude Juncker, EU foreign affairs chief Federica Mogherini and EU Enlargement Commissioner Johannes Hahn.
Germany
"The (German) government expects that the Turkish government will now seek respectful dialogue with all political and social forces in the country, after this tough election campaign," said Chancellor Angela Merkel in a joint statement with Foreign Minister Sigmar Gabriel.
France
France warned that if Turkey pushed ahead with a vote on restoring capital punishment, it would be a break with European values.
"The organisation of a referendum on the death penalty would obviously be a break with (the) values and engagements" that Ankara accepted when joining the Council of Europe, the French president's office said, referring to Europe's top rights watchdog.
Turkey is also seeking to join the EU, which has made clear that any restoration of the death penalty would scupper its accession hopes.
Switzerland
The Swiss foreign ministry urged Ankara to engage with 'No' voters in future policy-making "in the interests of stability and national unity in Turkey, using the opportunity to include voters in Turkish society who voted against the referendum."
Austria
Austrian leaders said the EU should end talks over Turkey's 30-year-old bid to join its ranks. "With what happened yesterday, (Turkey's) membership prospects are buried, in practical terms," said Chancellor Christian Kern. EU aid to Turkey to help it advance towards membership was now "obsolete," he added.
Foreign Minister Sebastian Kurz said the referendum meant "collaboration with the EU will be even more complex" and "we can no longer simply return to business as usual."
Council of Europe
Thorbjorn Jagland, secretary general of Europe's top rights watchdog, said Turkish leaders "should consider the next steps carefully ... in view of the close result."
It is of the utmost importance, said Jagland, "to secure the independence of the judiciary in line with the principle of rule of law enshrined in the European Convention on Human Rights."
The Council of Europe "stands ready to support the country in this process", Jagland added.
The Netherlands
"This constitutional change concentrates too much power in the hands of one person," Dutch Foreign Minister Bert Koenders said. "The European Union would have to keep a close eye on developments to follow," Koenders added.
Saudi Arabia
Turkey's ally Saudi Arabia offered "congratulations to President Erdogan of Turkey and the Turkish people on the success of the referendum regarding constitutional reforms," the official Saudi Press Agency reported, with the cabinet saying it hoped the vote would contribute to "more development success across the country".
NATO
A NATO official said that the constitutional referendum in alliance member Turkey "is a matter for the Turkish people".
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