Turkey's opposition has demanded the annulment of a contentious referendum that approved sweeping constitutional changes boosting President Recep Tayyip Erdogan's powers, claiming blatant vote-rigging had swung the result.
The European Union also urged a probe into the poll fraud claims after international observers voiced concerns, but both US President Donald Trump and Russian President Vladimir Putin yesterday called Erdogan to offer congratulations.
Critics fear the changes will lead to autocratic one-man rule, but supporters say they simply put Turkey in line with France and the United States and are needed for efficient government.
The 'Yes' camp won Sunday's poll with just 51.41 per cent of the vote but the result has been challenged, with angry protests staged in parts of Istanbul and other cities.
The changes, most of which are due to come into force after November 2019, are some of the most far-reaching in Turkey since Mustafa Kemal Ataturk established the modern state in 1923 on the ashes of the Ottoman Empire.
Bulent Tezcan, deputy leader of the main opposition Republican People's Party (CHP), formally requested that the Supreme Election Board (YSK) cancel the result.
The opposition is particularly incensed by a last-minute move by the YSK to accept ballot documents in envelopes without an official stamp.
"This is was a vote without legitimacy," Tezcan said.
Hundreds of people, some brandishing flares, marched through the anti-Erdogan Istanbul district of Besiktas in a new protest against the result, an AFP photographer said.
Meanwhile, in Kadikoy on the Asian side of the city, women banged drums and pans in a similar protest, with slogans like "We will not be scared, we will not be silent".
CHP chief Kemal Kilicdaroglu said the government and the YSK had "staged a coup against the national will".
The joint mission of the OSCE Office for Democratic Institutions and Human Rights (ODIHR) and the Parliamentary Assembly of the Council of Europe (PACE) said the YSK move on the stamps "removed an important safeguard".
They also said the campaign -- which saw the 'Yes' camp dominate the airwaves -- was conducted on an "unlevel playing field".
EU Commission spokesman Margaritis Schinas called on Turkish authorities "to launch transparent investigations into these alleged irregularities found by the observers".
In an interview with CNN, Erdogan denied claims Turkey was headed to dictatorship and that the new system was tailor-made for him.
"This is not a system belonging to Tayyip Erdogan. I am a mortal being, I can die any time," he told the channel.
The final results are due to be published in around 10 days and it is in this period that the YSK will consider the objections. Then the opposition can appeal to the constitutional court.