Turkey's referendum process to grant expansive new powers to President Recep Tayyip Erdogan was held on an "unlevel playing field", with campaigning restrictions and late procedural changes, monitors said.
Opposition events were obstructed and state resources misused, the Organisation for Security and Cooperation in Europe (OSCE) said.
In the referendum, voters gave sweeping new powers to President Erdogan, reported BBC.
The narrow vote was ruled valid by Turkey's electoral body, despite claims of irregularities by the opposition.
Erdogan's push for an executive presidency succeeded with 51 per cent voting for it.
"[The] referendum took place on an unlevel playing field and the two sides of the campaign did not have equal opportunities," the OSCE said in a statement.
"The campaign rhetoric was tarnished by some senior officials equating No supporters with terrorist sympathisers, and in numerous cases No supporters faced police interventions and violent scuffles at their events."
The main opposition Republican People's Party (CHP) has demanded a recount of 60 per cent of the votes.
Its deputy head said the result should be annulled altogether.
The pro-Kurdish Peoples' Democratic Party (HDP) also challenged the vote.
They criticised a decision by electoral officials to allow voting papers without official stamps to be counted, saying it opened the way for fraud.
But the head of Turkey's electoral body, Sadi Guven, said the unstamped ballot papers had been produced by the High Electoral Board and were valid.
He said a similar procedure had been used in past elections.
Three of Turkey's biggest cities -- Istanbul, Ankara and Izmir -- all voted "No" to the constitutional changes, BBC reported.
Opposition supporters took to the streets of Istanbul to bang pots and pans, a traditional form of protest, in a series of noisy demonstrations.
Meanwhile, flag-waving supporters of Erdogan celebrated as their President praised them for their "historic decision" that could keep him in office until 2029.
With 99.97 per cent of ballots counted, the "Yes" campaign had won 51.41 per cent of the votes cast, while "No" had taken 48.59 per cent.
The turnout was said to be as high as 85 per cent.
Deputy Prime Minister Mehmet Simsek said there would be no early elections following the result.
Meanwhile, the German government on Monday urged Turkey to hold "respectful dialogue" with different political factions in the country after the narrow referendum win backing constitutional changes.
In a joint statement, German Chancellor Angela Merkel and Foreign Minister Sigmar Gabriel said the outcome of the vote had indicated a "deeply divided" Turkish society.
Austria's Foreign Minister Sebastian Kurz said the outcome of Sunday's referendum went against EU principles.
The final referendum results will be released within 12 days.
--IANS
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