Turkey's referendum campaign was "unequal", with opponents suffering restrictions and state resources being misused, international monitors say.
Late changes in ballot counting marred the vote and removed a key safeguard, they added. In the referendum, voters narrowly gave sweeping new powers to President Recep Tayyip Erdogan.
The vote was ruled valid by Turkey's electoral body, despite claims of irregularities by the opposition. Mr Erdogan's push for an executive presidency succeeded with 51.4% voting for it.
Despite saying that the voting day was "well administered", the Organization for Security and Co-operation in Europe (OSCE) and the Council of Europe criticised the campaign, saying:It was an "unlevel playing field" and the two sides of the campaign "did not have equal opportunities"It was unbalanced due to the active involvement of the president and several senior officialsIt was tarnished by a number of senior officials equating No supporters with terrorist sympathisersAdministrative resources were misusedUnder the state of emergency, essential fundamental freedoms were curtailed
They also criticised a late change by electoral officials that allowed voting papers without official stamps to be counted. They said this move "removed an important safeguard and were contested by the opposition.
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.
President Erdogan said on Monday that his side had prevailed despite what he called the attacks of the "Crusaders in the West".
"We will march to November 2019 faster and stronger," he said, referring to the next general election.
"But I should express again that we have much to do. We all are aware of this. Because it was a fight against everyone. The crusaders' ideology in the West and their servants here attacked us. But we didn't give up. We stood up as a nation."
But the main opposition Republican People's Party (CHP) has demanded a recount of 60% of the votes. Its deputy head said the result should be annulled altogether.
The pro-Kurdish Peoples' Democratic Party (HDP) also challenged the vote.
Right after the result, Mr Erdogan said Turkey could now hold a referendum on bringing back the death penalty - a move that would end Turkey's EU negotiations.(Source: BBC)