"We thank the OSCE Referendum Observation Mission for its important work and note the concerns detailed in its preliminary assessment of the conduct of the vote and campaign".
The HDP said there were indications of a manipulation amounting to three or four percentage points while deputy CHP leader Erdal Aksunger said up to 60 percent of the ballot boxes could be appealed.
In their petitions, the residents of Ankara said the decision and other reported irregularities were in open violation of the law. Everyone and all sections - and the main opposition party in particular - must show respect.
Dundar also said he viewed Turkey's referendum as similar to Brexit, in that both were, in a sense, about breaking away from Europe.
Toner added that the USA government was looking "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, such as under the Helsinki Charter".
Overnight, sporadic protests by disgruntled "No" voters erupted in parts of Istanbul, with demonstrators banging pots and pans to voice their discontent.
Council of Europe observers say those irregularities could have changed the outcome of the vote.
In the coming months, Erdogan hinted at holding a second referendum on the European Union accession process, in which Turkish voters would decide whether to end long-stalled negotiations.
Opposition CHP head Kemal Kilicdaroglu accused the electoral board of bias and of favoring the governing party.
The constitutional change means the the victor of the 2019 presidential election, likely to be Erdogan himself, will assume a position of far greater authority. Critics have warned that passing the constitutional changes would be a serious blow to democracy and benefit Erdogan and his purge of political enemies including media that is critical of his rule. "We have an unequivocal rejection of the death penalty".
(AP Photo/Cansu Alkaya). Supporters of pro-'no' vote chant slogans as they protest in Istanbul, against the referendum outcome, early Monday, April 17, 2017.
Erdogan reiterated his readiness to restore the death penalty at several appearances on Monday, which would effectively end Turkey's decades-long quest to join the EU.
"It is the reddest of the red lines", said European Commission spokesman Margaritis Schinas.
Global election observers and opposition parties have reported numerous voting irregularities during the vote which President Recep Tayyip Erdogan won by a narrow margin.
"It's a tremendously successful job", he said past year in a radio interview with Steve Bannon, then of Breitbart News, who would later become his campaign chairman before joining the Trump White House.
The narrow victory of a historic referendum to centralize power in the hands of Turkish President Recep Tayyip Erdogan has been tarnished with allegations of fraud.
Ahead of the referendum, tensions arose between the European Union and Turkey, particular in European Union member states with large Turkish populations like Germany.
The referendum approves 18 constitutional amendments to replace Turkey's parliamentary system with a presidential one.
Reflecting a cooling of relations between Ankara and Moscow following Erdogan's "full support" for the USA missile attack on a Syrian air base last week, there has thus far been no congratulatory call from Russian President Vladimir Putin.