A Council of Europe (CoE) electoral observer said on Tuesday that up to 2.5 million votes could have been manipulated during Turkey's presidential referendum.
Several institutions, including monitors from the Organisation for Security and Co-operation in Europe (OSCE), said Sunday's referendum to greatly expand the President's power did not meet democratic standards.
"There is a suspicion that up to 2.5 million votes could have been manipulated", Alev Korun, an Austrian Green politician who was on the observer mission at the CoE, said. Ealier the opposition parties in Turkey has formally requested the referendfum to be annulled.
The main opposiiton party had previously demanded a partial recount of the 2.5 million ballot papers that were accepted by the Supreme Electoral Council as valid despite being unstamped.
Any official challenge regarding the 2.5 million papers would be a threat to the "Yes" camp, which edged past "No" voters by a slim margin of 1.25 million.
Turkish President Recep Tayyip Erdogan, whose Justice and Development Party (AKP) penned and campaigned for a "Yes" vote, has already claimed victory and later received a congratulatory phone call from US President Donald Trump.
Korun said she herself did not witness any irregularities in the voting process, but that two of her colleagues had been denied entry to a polling station in the predominantly Kurdish city of Diyarbakir, in the country's restive southeast.
The "Yes" campaign claimed 51.4 percent of a vote in favour of an 18-point overhaul of the constitution that is to scrap the role of Prime Minister and hand almost all executive power to the President.
But many officials have raised their concerns about the environment in which the vote was held.
On Monday, head of the OSCE observation mission for the referendum, Tana de Zulueta, said there was a lack of impartiality in the run-up to Sunday's vote as the "Yes" campaign was given the chance to dominate media coverage, while the other side was restricted and many journalists were arrested.
Several governments in Europe have called on Erdogan to prove that the election was conducted fairly and cleanly by cooperating fully with international observers.
With inputs from agencies