Turkey's electoral board confirmed the "yes" victory in yesterday's referendum and said the final results would be declared in 11-12 days. The state-run Anadolu Agency said the "yes" vote stood at 51.41 per cent, while the "no" vote was 48.59 per cent.
The margin fell short of the sweeping victory Erdogan had sought in the referendum. Nevertheless, it could cement his hold on power in Turkey and is expected to have a huge effect on the country's long-term political future and its international relations.
The 18 constitutional amendments that will come into effect after the next election, scheduled for 2019, will abolish the office of the prime minister and hand sweeping executive powers to the president.
Opponents had argued the constitutional changes would give too much power to a man who they say has shown increasingly autocratic tendencies.
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Opposition parties complained of a number of irregularities in the voting, including an electoral board decision to accept as valid ballots that did not bear the official stamp.
Legislator Utku Cakirozer told The Associated Press that his Republican People's Party would on Monday file objections to results at local electoral board branches, before taking their case to the Supreme Electoral Board.
Cakirozer said: "At the moment this is a dubious vote." The country's pro-Kurdish party said it may take the case to the European Court of Human Rights if the electoral board does not reverse his decision and nullify the ballots lacking the official stamps.
Electoral board head Sadi Guven rejected opposition claims of foul play, saying none of the ballot papers that were declared valid was "fake" or fraudulently cast. Guven said the decision was taken so that voters who were by mistake given unstamped ballot papers would not be "victimized."
"The ballot papers are not fake, there is no (reason) for doubt," Guven said.
In Istanbul, hundreds of demonstrators opposed to the amendments marched in a central neighborhood late Sunday, clanging pots and pans.