Erdogan has lashed back at the OSCE worldwide monitors' initial findings, telling them to "know their place", Reuters reports.
The "yes" camp won 51.41 percent in Sunday's referendum on a new presidential system and the "no" camp bagged 48.59 percent of the votes, according to near-complete results released by the election authorities.
In Istanbul, hundreds of "no" supporters demonstrated in the streets last night, chanting "thief, murderer, Erdogan" and banging pots and pans.
"The tight referendum result shows how deeply divided Turkish society is, and that means a big responsibility for the Turkish leadership and for President Erdogan personally".
Declining to congratulate the Turkish president, the European Union executive's second official response since the vote instead focused on observer findings that the vote was skewed in Erdogan's favor without a proper legal framework and with late changes in ballot counting.
(AP Photo/Emrah Gurel). A supporter of the "no" vote lights a flare during a protest in Istanbul, against the referendum outcome, Monday, April 17, 2017.
Schinas said that such caution was needed not only because of the observers' report but also because of the closeness of the result and what he called the "far-reaching" implications of the constitutional amendments.
"While the parliament is required to have two-thirds majority to amend the constitution, [it is strange] to have absolute majority of the people to change the governing style of the Republic of Turkey", one Ankara resident told VOA.
The deputy leader of country's largest opposition party, the Republican People's Party (CHP), on Monday called for the results of the referendum to be annulled. The group was also concerned that the election board accepted unstamped ballots, in violation of Turkish law.
"The German government ... respects the right of Turkish citizens to decide on their own constitutional order", they said in a statement.
The head of the board, Sadi Guven, strongly defended his decision to allow the controversial ballots, citing high demand for ballots and saying similar procedures had been followed in the past.
The report by the global referendum observer mission said the legal framework for the vote was inadequate for a genuinely democratic referendum.
Mr Erdogan has rejected criticism from worldwide monitors who said he had been favoured by an "unequal campaign".
The referendum itself was portrayed outside Turkey as an attempt by Erdogan to make himself a dictator.
The Commission said that any legislation bringing back the death penalty to Turkey, as pledged by Erdogan during his campaign, would certainly end Ankara's European Union membership bid.