Irregularities could have changed the outcome of the Turkish referendum which ended in a tight 'Yes' vote for greater presidential powers, says Alev Korun, an Austrian member of the Council of Europe observer mission.
"There is a suspicion that up to 2.5 million votes could have been manipulated," the Austrian member of parliament told ORF radio on Tuesday.
"This is about the fact that actually the law only allows official voting envelopes. The highest election authority decided however, as it were against the law, that envelopes without official stamp should be admitted," Korun added.
Turkish President Tayyip Erdogan has dismissed criticisms of Sunday's vote.
Korun also said police in the majority Kurdish town of Diyarbakir hindered two of her colleagues from entering voting locations. She pointed to videos on social media, which appear to show people voting more than once, although this still had to be proven.
"These complaints are to be taken very seriously and they are, in any case, of such an extent that they would turn around the outcome of the vote," Korun said.
The pro-Kurdish opposition Peoples' Democratic Party said it presented complaints about unstamped ballots affecting three million voters, more than twice the margin of President Tayyip Erdogan's victory.
The Union of Turkish Bar Associations (TBB) said a last-minute decision by Turkey's electoral board to allow unstamped ballots was clearly against the law and may have impacted the results.
"With this illegal decision, ballot box councils (officials at polling stations) were misled into believing that the use of unstamped ballots was appropriate," the TBB said in a statement.
"The YSK (electoral board) announcement, which is clearly against the law, has led to irregularities, and the prevention of records that could uncover irregularities from being kept," it said, adding that it expected the YSK to now realise its constitutional responsibilities in evaluating complaints.
