Turkeyinfo-icon's opposition on Tuesday 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 Unioninfo-icon also urged a probe into the poll fraud claims after international observers voiced concerns, but USinfo-icon President Donald Trump showed no such scruples, calling Erdogan to offer his congratulations.

The main opposition Republican People's Party (CHP) deputy leader Bulent Tezcan 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 an election without legitimacy," Tezcan said after delivering the complaint to YSK headquarters in Ankara, claiming there was an organised campaign to "steal" the people's will. "There is only one thing to do ... and that's to annul the referendum," he said.

CHP chief Kemal Kilicdaroglu also said the government and the YSK had "staged a coup against the national will" in a posting on Twitterinfo-icon. He said 'No' would have emerged victorious were it not for the YSK's changes. "We won't stop until we find justice," he said.

The joint mission of OSCE Office for Democratic Institutions and Human Rightsinfo-icon (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 dominating the airwaves -- was conducted on an "unlevel playing field".

But Erdogan rebuked the OSCE mission, telling the monitors: "know your place", and saying Turkey had no intention of paying any attention to the report.

EU Commission spokesman Margaritis Schinas also called on the Turkish authorities "to launch transparent investigations into these alleged irregularities found by the observers".

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 courtinfo-icon.

In a blow to the president's prestige, the 'No' campaign notched up the most votes in Turkey's three biggest cities: Istanbul, Ankara, and Izmir. Analysts have said the 'No' camp's performance was impressive especially given that the election was held under a state of emergency first imposed after the failed July coup.

"Under the conditions of the emergency and despite the pressures, our country came out and took control of the future," said CHP leader Kilicdaroglu.

The Hurriyet daily said a major cabinet reshuffle was now in the offing, with changes expected in some 10 cabinet posts.

Meanwhile, parliament agreed to extend the state of emergency -- already in place for nine months -- for another three months to July 19.