The result of the referendum that grants sweeping new powers to the president of Turkey is valid, the head of the electoral body says.
Sadi Guven was speaking after the main opposition Republican People's Party (CHP) cited irregularities, including the use of unstamped ballot papers.
President Recep Tayyip Erdogan's push for an executive presidency succeeded with just over 51% of the vote.
The win was met with both celebrations and protests across Turkey.
The CHP has demanded a recount of 60% of the votes.
But Mr Guven told reporters on Monday 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.
Opposition supporters took to the streets of Istanbul to bang pots and pans - a traditional form of protest - in a series of noisy demonstrations.