International election observers delivered a scathing verdict on Turkey's referendum, saying the contest was an "unlevel playing field" and that the election committee broke the law with last-minute changes to voting rules.

The blunt assessment by the Organisation for Security and Co-operation in Europe (OSCE) drew a furious response from President Recep Tayyip Erdogan, who told the observers to "know your place".

Following his narrow victory, Erdogan vowed he would forge ahead with plans to reshape Turkey's constitution despite calls from the main opposition party for the results to be voided because of alleged voting irregularities.

"We will continue down our road," he said.

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"This country held the most democratic elections that have ever been seen in any other country in the West."

The OSCE team said the referendum "took place on an unlevel playing field and the two sides of the campaign did not have equal opportunities".

Both the observers and the Turkish opposition focused on a decision by Turkey's high electoral board - made after voting had already started - to count ballots even if they had not been officially stamped.

The decision "significantly changed the ballot validity criteria, undermining an important safeguard and contradicting the law," the OSCE said.

Opposition parties said that 1.5 million unstamped ballots had been illegally counted - enough to swing an election that had been decided by about 1.38 million votes, according to the state news agency.

The OSCE said the No campaign had been starved of coverage by state-run media and faced violence from both police and Yes supporters.

It also criticised Mr Erdogan's fiery language, saying: "The campaign rhetoric was tarnished by a number of senior officials equating No supporters with terrorist sympathisers."

The OSCE has no power to force a re-run of the election and Turkey's electoral committee showed no sign of backing away from its decision to certify Mr Erdogan's Yes camp as the winners.

"The ballot papers are not fake, there is no reason for doubt," said Sadi Guven, chairman of the electoral board.

Erdogan signalled his defiance of Europe by saying he planned to move forward with plans to reinstate the death penalty in Turkey - a move that would end any lingering hopes of Turkey joining the EU.

France warned the move would be a "break with the values" of Europe.

The Turkish president also suggested he might hold a referendum on whether to continue with Turkey's application to join the EU, which has been stalled for years.

"For 54 years, what did they make us do at the EU's door? Wait," he told supporters in Ankara.

Supporters of the 'No' vote chant slogans during a protest in Istanbul. Photo / AP
Supporters of the 'No' vote chant slogans during a protest in Istanbul. Photo / AP

Several hundred young people took to the streets in Istanbul, banging pans to demonstrate against the result, but the mood among the opposition was generally subdued and there was no sign of mass protests forming.

"You can see on social media the shenanigans and the vote stealing that went on," said Yakup Yeldiz, a 21-year-old student. "The high electoral board has acted like this is a coup."

But he predicted there would not be widespread protests against the result, especially in light of the state of emergency that was declared after last year's failed coup attempt, which gives police widespread powers to detain people.

"When people on the right are angry, they reach for weapons. When people on the left are angry, they reach for pots and pans," he said.