They also cited inappropriate use of administrative resources to support Erdoğan's position and a lack of balanced information for voters to make informed choices.
On the face of it, even if it were legitimate, the referendum affirms Erdogan's intention to transform Turkey's cumbersome system of parliamentary checks and balances and judicial independence into a neo-Ottoman presidential republic with neither.
The decision upholds the result of the landmark referendum, which has polarised Turkey in the last few months and highlighted the splits in a society that is still reeling from a coup attempt past year and repeated terror attacks by Islamic State and Kurdish separatists. Other changes are to be implemented sooner, including scrapping a requirement that the president not be a member of any political party.
He can certainly go on winning elections for a while because his support is rock-solid among the half of the population who felt oppressed by the secular state created by Ataturk nearly a century ago.
The election results have led to protests in Istanbul and other cities.
Opposition parties have complained of a series of irregularities, particularly an electoral board decision to accept ballots without official stamps, as required by Turkish law. The area is home to many opponents of the president.
But rather than bringing back state-sanctioned murder, Erdogan's top domestic priority should be to reunite the polarized country, divided between a secular class and the more ultra-nationalist and conservative faction that supports him.
Erdogan and senior government officials say the vote is a settled matter. "Anyone who wants to live here with us must adapt", he added.
Similar protests were held across Istanbul. But Erdogan holds a strong hand in any dealings with Europe: Turkey previous year agreed to limit the number of refugees from Syria and other conflict zones heading to Europe, but it keeps threatening to loosen its controls. In return, right-wing state security forces protected their organized criminal operations, including drug trafficking.More news: US Treasury says China does not manipulate its currency
The referendum took place under a state of emergency that was declared following a failed military coup last summer.
To fix his relationship with Europe and the United States, Erdogan needs to reinstate press freedom, free imprisoned journalists and parliamentarians, stop publicly vilifying his opponents and put an end to the Nazi references he used to describe the German and Dutch governments.
(AP Photo/Petros Karadjias). Supporters of the "no" vote, (Hayir in Turkish) chant slogans during a protest against the referendum outcome, in Istanbul, Tuesday, April 18, 2017. The call was mainly focused on Syria, the official told CNN. Some are said to provide evidence of vote-stuffing.
'Now Erdogan will have to rule the part of Turkey most open to the world, with the highest cultural production, export capacity, tourism revenue and industrial output, with a constitution approved by its most introvert part, ' he wrote.
In recent weeks, the pro-Kurdish party HDP urged voters to reject the amendments.
The head of Germany's main industrial group says the referendum in Turkey could hurt economic ties with European countries. "It would be a false conclusion to undermine our ideas of a modern Europe because of unwanted election results", he said. He told reporters, "It is unacceptable for the main opposition party not to acknowledge results which the public has already acknowledged".
The CHP said in a petition for annulment that the decision by the Supreme Board of Elections to include the unsealed ballots violated Turkish law.
Bulent Tezcan, deputy chairman of the Republican People's Party, announced the move at the Ankara offices of the electoral board.
"We look to the government of Turkey to protect the fundamental rights and freedoms of all its citizens - regardless of their vote on April 16 - as guaranteed by the Turkish constitution and in accordance with Turkey's global commitments", the State Department said in a statement.