With 98 per cent of the votes counted many now fear Erdogan has been given permission to shape Parliament to his wishes.
Erdogan, 63, initially sounded conciliatory in his remarks, saying the result was a victory not just for those who voted "yes", but for "the whole 80 million, the whole of Turkey".
Turkey's main opposition Republican People's Party (CHP) had been campaigning for a "no" vote, against the proposed changes.
"Believe me, this election is not over", deputy CHP leader Erdal Aksunger told CNN Turk, quoted by the Dogan news agency.
The referendum was held under a state of emergency that has seen 47,000 people arrested in an unprecedented crackdown after a failed military putsch against Erdogan in July a year ago.
Concerns over the handling of the referendum forced the EU deploy a delegation of 20 impartial observers to make sure it adhered to European standards.
The referendum on constitutional amendments in Turkey on Sunday was not in compliance with the standards of the Council of Europe, worldwide observers from OSCE Office for Democratic Institutions and Human Rights (ODIHR) said in a statement.
Some people in Turkey are concerned that the result will mean an increase in authoritarianism, as Erdoğan will gain additional powers as Turkey's president - he has already touted bringing back the death penalty.
It would dispense with the prime minister's post and centralise the entire executive bureaucracy under the president, giving Erdogan the direct power to appoint ministers.
Blues game away from series sweep
Bortuzzo (upper body injury), who missed Game 1 and the last five games of the regular season, replaced rookie Jordan Schmaltz. Guentzel completed the hat trick off a power move and pass from Crosby for the game-winner, his second of the series.
Sunday's vote ended all debate on forging a stronger presidency, Erdogan said, vowing that implementation of the reforms would begin straight away.
One objection that European Union members would have to ascension would be Turkey's possible re-implementation of capital punishment, which has not been used by the courts since 1984 and officially abolished in 2004.
German Chancellor Angela Merkel in a joint statement with Foreign Minister Sigmar Gabriel that Berlin expects Ankara will now "seek respectful dialogue with all political and social forces in the country".
But Preda added: "It's not our role to say what is the level of fraud or (comment on) the other allegations made by the opposition".
Once the president's referendum victory was announced, Erdogan supporters swarmed the streets and started "hunting" down his protesters.
They also said the restrictions on media outlets and arrests of journalists ensured the yes campaign backed by President Recep Tayyip Erdoğan and the ruling party received the lion's share of coverage, tilting the odds in their favour.
There has been some speculation that Erdogan could call a new election so that his new powers could take effect right away, but Simsek dismissed that.
However, officials said the elections would be held in 2019 as scheduled.
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