The Turkish referendum fell short of European standards, international observers say.
Cezar Florin Preda, head of the delegation from the Parliamentary Assembly of the Council of Europe that monitored the vote, cited an inadequate legal framework and late changes in ballot counting.
Preda said "generally speaking the referendum fell short of CoE standards ... it did not provide for a truly democratic process."
He made the comments at news conference in Ankara. Turkey is a member of the Council of Europe.
Preda was joined by Tana de Zulueta, head of another team of international observers.
Turkey's High Electoral Board made a last-minute decision on Sunday to count ballots that had not been stamped by officials.
Turks on Sunday voted by a narrow 51.4 per cent margin to change their constitution and grant President Tayyip Erdogan sweeping new powers. The main opposition party has demanded the result be nullified, saying the voting was marred by irregularities.