Turkish President Recep Tayyip Erdogan greets supporters as he arrives at Eyup Sultan mosque in Istanbul, Turkey on April 17.
Erdogan has long said the changes were needed to end chronic instability that plagued the country over decades when the military repeatedly tried to seize power from weak civilian governments.
"Before counting, ballots, envelopes and number of voters are crosschecked", Guven said.
Turkey's main opposition party on Monday announced that it would be seeking annulment of the referendum on constitutional reforms.
The president survived a coup attempt past year and responded with a crackdown, jailing 47,000 people and sacking or suspending more than 120,000 from government jobs such as schoolteachers, soldiers, police, judges or other professionals.
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He added, "The change in the constitution which was accepted with the highest turnout in Turkish history will benefit our nation".
Following decisions by German and Dutch authorities to block pro-Erdogan rallies in their countries ahead of the referendum, the Turkish president characterized his European opponents as the "grandchildren of Nazism".
He said over 1,5 million invalid votes were counted as "yes".
The European Council recognized Turkey as a candidate for full membership in 1999 and negotiations were opened in 2005, but have since been suspended over concerns of authoritarianism in Turkey.
"The organisation of a referendum on the death penalty would obviously be a break with (the) values and engagements" accepted by Turkey when it joined Europe's top rights watchdog, the Council of Europe, the presidency said. The official results are expected within 12 days.
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He is a popular figure among Palestinians, with polls suggesting he could win the Palestinian presidency. The strike coincides with Palestinian Prisoners Day , which is usually marked on 17 April.
Sunday's vote grants sweeping powers to Erdogan, including the ability to directly appoint ministers and intervene in the judiciary. German Chancellor Angela Merkel said that the tight result "shows how deeply divided Turkish society is" and said the result entailed "a big responsibility for the Turkish leadership and President Erdogan personally".
As the world wakes to news of the Turkish people voting "Yes" in a in a slews constitutional changes set to dramatically change the face of governance in the geo-politically important state, European leaders have urged their colleagues to "keep calm" and their Turkish counterparts to continue to work toward a national consensus and to await for the official results.
Opposition parties have argued that the changes, which come into effect after the 2019 presidential election, give too much power to the office.
Under the proposals, the office of prime minister would be abolished, allowing the president to draft the budget, declare a state of emergency and issue decrees overseeing ministries without parliamentary approval.
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Whether it resonates beyond Tax Day - which is typically April 15, though the IRS is pushing the deadline to Tuesday - is unclear. Trump may have seen some of the protesters Saturday as he went by vehicle from Mar-a-Lago to his golf club in West Palm Beach.