US President Donald Trump called his Turkish counterpart Recep Tayyip Erdogan on Monday to congratulate him on winning the referendum boosting his powers, Turkish state media reported.

“Trump called Erdogan tonight (Monday) and congratulated him on his success in the referendum,” Turkish presidential sources said, quoted by state-run news agency Anadolu after Erdogan won over 51 per cent in Sunday’s vote.

The reaction from Trump contrasts with EU leaders who have been reserved in their reaction to the narrow victory while monitors expressed concern over the “unlevel playing field”.

The ‘yes’ camp won 51.41 per cent in Sunday’s referendum and ‘no’ 48.59, according to near-complete results released by the election authorities.

But the opposition immediately cried foul over alleged violations, claiming that a clean vote would have made a difference of several percentage points and handed them victory.

The main opposition Republican People’s Party (CHP) and the pro-Kurdish Peoples’ Democratic Party (HDP) said they would challenge the results from most of the ballot boxes due to alleged violations.

“There is only one decision to ease the situation in the context of the law – the Supreme Election Board (YSK) should annul the election,” the Dogan news agency quoted CHP deputy leader Bulent Tezcan as saying.

Returning in triumph to his presidential palace in Ankara, Erdogan addressed thousands of supporters gathered outside, telling monitors who criticised the poll: “Know your place.”

Showing no sign of pulling his punches, Erdogan said Turkey could hold further referendums on its EU bid and re-introducing the death penalty.

The opposition was particularly incensed by a decision by the YSK to allow voting papers without official stamps to be counted, which they said opened the way for fraud.

The referendum has no “democratic legitimacy”, HDP spokesman and MP Osman Baydemir told reporters in Ankara.

The opposition had already complained of an unfair campaign that saw the ‘yes’ backers swamp the airwaves and use up billboards across the country in a saturation advertising campaign.

The referendum campaign was conducted on an “unlevel playing field” and the vote count itself was marred by the late procedural changes that removed key safeguards, international observers said.

“The legal framework ... remained inadequate for the holding of a genuinely democratic referendum,” the OSCE Office for Democratic Institutions and Human Rights (ODIHR) and the Parliamentary Assembly of the Council of Europe (PACE) monitors said in a joint statement.

“Late changes in counting procedures removed an important safeguard,” said Cezar Florin Preda, the head of the PACE delegation, referring to a move by the election authorities to allow voting documents without an official stamp.

Erdogan’s victory was far tighter than expected, emerging only after several nail-biting hours late Sunday which saw the ‘no’ result dramatically catch up in the later count.

Turkey’s three largest cities – Istanbul, Ankara and Izmir – all voted ‘no’ although ‘yes’ prevailed in Erdogan’s Anatolian heartland.

The new system is due to come into effect after elections in November 2019.

The new system would dispense with the office of prime minister and centralise the entire executive bureaucracy under the president, giving Erdogan the direct power to appoint ministers.

It would also mean that Erdogan, who became president in 2014, could seek two more five-year terms, leaving him in power potentially until 2029.

Throughout the campaign, Erdogan launched bitter attacks on the European Union, accusing member states of behaving like the Third Reich in failing to allow his ministers to campaign among expats.

The initial reaction from Turkey’s Western allies was far from ebullient, with top EU officials saying Turkey had to find the “broadest possible” agreement on the changes in view of the closeness of the result.

Merkel said Berlin expects Ankara will now “seek respectful dialogue with all political and social forces in the country”.

In an indication more strife with Brussels could be in the offing, Erdogan said he would now hold talks on reinstating capital punishment, a move that would automatically end Turkey’s EU bid.

If the opposition failed to support such a bill, he said another referendum could be held on reinstating the death penalty.

French President Francois Hollande also urged dialogue ­between the political factions.

“It’s up to the Turks and them alone to decide on how they ­organise their political institutions, but the published results show that Turkish society is divided,” Hollande’s office said.

There was talk Erdogan would call an election so his powers could take effect. However, officials said the elections would be held in 2019 as scheduled.

If Turkey were to hold a referendum on bringing back capital punishment it would be a break with European values, the Hollande’s office warned.

Saudi Arabia, meanwhile, praised the “success” of a referendum.

The cabinet, at its weekly meeting, voiced “congratulations to President Erdogan of Turkey and the Turkish people on the success of the referendum regarding constitutional reforms”, the official Saudi Press Agency reported. The cabinet said it hoped the vote would contribute to “more development success across the country”.

Additional reporting by Reuters