Turkey opposition threatens legal challenge to referendum result

"At the moment it is impossible to determine how many such votes there are and how many were stamped later. This is why the only decision that will end debate about the legitimacy [of the vote] and ease the people's legal concerns is the annulment of this election by the YSK," he told reporters.

If appeals to Turkey's municipal election authorities, the YSK and the country's supreme court are unsuccessful, Tezcan said, the CHP will bring the case to the ECHR or other similar international bodies.

"We observed the misuse of state resources, as well as the obstruction of 'No' campaign events," it 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."

Despite this, the OSCE said there were no major problems, "except in some regions" on referendum day.

President Recep Tayyip Erdogan's drive to increase his executive powers as president succeeded with 51.4 percent voting in favour of it.

The new presidential system will dispense with the office of prime minister and centralise the entire executive bureaucracy under the president, giving Erdogan the direct power to appoint ministers.

It is expected to come into force after the elections in November 2019. Erdogan, who became president in 2014 after serving as premier from 2003, could then seek two more five-year terms.

Supporters see the new system as an essential modernisation step for Turkey that will remove the risk of the political chaos that blighted the 1990s and is blamed for the 2000-2001 financial crisis.

Opponents fear it risks granting Erdogan authoritarian powers and allow him to ride roughshod over key institutions like the judiciary and parliament.