The referendum in Turkey did not live up to standards set by the Council of Europe, according to a representative from the European human rights organisation’s observer mission.
Officials pointed to an inadequate legal framework and last-minute changes in counting the ballots, as well as a "skewed pre-vote campaign" in favour of the "yes" vote and intimidation of the opposition.
The remark followed 51.4 per cent of Turks voting in favour of changing their constitution and granting President Tayyip Erdogan sweeping new powers, including re-introducing the death penalty and personally appointing ministers and half of the judicial body.
Critics said the referendum question was not listed on the ballot and that the voting was impacted by irregularities.
The margin of victory was narrow and opposition leaders have demanded that the result be nullified.
Cezar Florin Preda, head of the delegation from the Parliamentary Assembly of the Council of Europe which monitored the vote, spoke of the referendum at a news conference in Ankara.
Tana de Zulueta, head of another team of international observers, said the referendum missed the mark on international standards.
The High Electoral Board in Turkey made a late decision on the night of the referendum to count ballots that had not been stamped by officials, which observers said undermined important safeguards against fraud.
Bulent Tezcan, deputy chairman of Turkey's main opposition Republican People's Party, claimed the decision was made after it appeared the "no" votes were ahead of the "yes" votes, and that initial counting of the ballots happened in secret.
But the head of that Board, Sadi Guven, defended the process, saying all the ballot papers that were counted were not fake or cast fraudulently.
Turkey's main opposition party is preparing to contest the results, which were released by the state-run Anadolu Agency.
German Chancellor Angela Merkel said she supported the right of the Turkish people to hold a referendum, but warned that Mr Erdogan should have a "respectful dialogue" with the other parties following the narrow results.
French leader Francois Hollande echoed the call, but added that France will follow observers' guidance as to whether the vote was free and fair.
The referendum on Sunday night followed remarks from the Organisation for Security and Cooperation in Europe, of which Turkey is a member, that the conditions for the vote did not appear "fair".
The OSCE cannot sanction Turkey, it can only make recommendations.
It is expected that the Turkish President will extend the country's controversial state of emergency, due to expire on 19 April, for another three months under the pretext of cracking down on an extremist Muslim network.
Critics say the move was designed to shut down the opposition, with more than 40,000 arrests and an estimated 100,000 people removed from their government posts.
Mr Erdogan, who could now be in power for another decade, initially claimed the result was a victory for both sides, for "the whole 80 million" Turks. Later in Istanbul, he changed his tone.
"There are those who are belittling the result. They shouldn't try, it will be in vain," he said. "It's too late now."
Agencies contributed to this report.
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