"These violations contravene OSCE commitments, Council of Europe standards and other global obligations regarding freedom and equality in the campaign", it said, referring to the Organization for Security and Cooperation in Europe, an intergovernmental body that regularly reviews human rights practices in Europe and beyond.
Opponents had argued the constitutional changes would give too much power to a man who they say has shown increasingly autocratic tendencies.
Returning in triumph to his presidential palace in Ankara, Erdogan addressed thousands of supporters gathered outside, telling monitors who criticised the poll: "Know your place".
The cautious response to the referendum results from European Union capitals highlights, however, the importance to the bloc of ensuring the continued success of a migration deal with Turkey, under which the country acts as a buffer to those seeking to settle in the bloc in return for economic aid.
Tezcan said he would if necessary go to Turkey's constitutional court - one of the institutions that Erdogan would gain firm control over under the constitutional changes, through the appointment of its members.
Election authorities said preliminary results showed 51.4 percent of voters had backed the biggest overhaul of Turkish politics since the founding of the modern republic.
She said the referendum contravened Turkey's commitments to the standards of the Organization for Security and Cooperation in Europe and the Council of Europe "regarding freedom and equality in the campaign".
Erdoğan has been defiant about any notion of a recount in the vote, which opposition members have called for after the three million unstamped votes were accepted by the Supreme Electoral Board (SBE).
"The United States continues to support Turkey's democratic development, to which commitment to the rule of law and a diverse and free media remain essential", he said.
Jordan Hasay Calls on Memory of Her Mom During Her First Marathon
Hasay said she was honored running with Linden and gathered energy from the chants of "USA, USA" toward the finish. Her boyfriend, who was running with her, managed to shove the official out of the way and Ms Switzer kept running.
"In numerous cases, "no" sympathizers faced police interventions and violent scuffles at their events", she said.
"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 mission observation.
But a crackdown by Erdogan since a failed coup last July has been condemned in European capitals.
By law, for a vote to be considered valid, the ballot and the vote must bear official stamps.
Turkey's Supreme Election Board's (YSK) decision to deem unsealed ballots valid in the referendum had lifted significant assurances and that it was against the law, according to global observers.
Turkish opposition parties have called for the referendum to be annulled, official results will be released.
The combined effect seriously tarnished President Recep Tayyip Erdogan's close victory in the vote on Sunday, which replaces a century-old parliamentary democracy with a "presidential system" that greatly expands the president's authority and would allow Erdogan to serve for another 15 years in office.
Erdogan says concentrating power in the hands of the president is vital to prevent instability.
But Erdogan's victory was far tighter than expected, emerging only after several nail-biting hours late Sunday which saw the "No " result dramatically catch up in the later count.
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