ANKARA - Turkey's opposition Monday called for the annulment of a referendum giving President Recep Tayyip Erdogan sweeping new powers, as international monitors voiced concern over the campaign and vote count.

The referendum was seen as crucial not just for shaping the political system of Turkey but also the future strategic direction of a nation that has been a NATO member since 1952 and an EU hopeful for half a century. Showing no sign of pulling his punches after the referendum victory, Erdogan sparked new alarm in EU capitals by saying Turkey's next plebiscite could be on re-introducing the death penalty.

The 'Yes' camp won 51.41 percent in Sunday's referendum and 'No' 48.59, according to complete results released by election authorities.

Erdogan, who opponents fear could now assume one-man rule, returned to Ankara from Istanbul, welcomed by thousands lining the roads as he drove in triumph aboard the presidential bus.

In a brief speech from the airport, Erdogan congratulated cheering supporters for "standing tall" in the face of the "crusader mentality" of the West.

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.

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.

But Turkey's foreign ministry his back at the assessment, saying it was "biased" and "prejudiced" and adding it was "unacceptable" to say the poll did not reach democratic standards.

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 'Yes' was victorious but the people have sent messages to the government and opposition that need to be carefully considered," wrote pro-government Hurriyet columnist Abdulkadir Selvi.

The new system is due to come into effect after elections in November 2019 although Erdogan is expected to rapidly rejoin the ruling Justice Development Party (AKP) he founded but had to leave when he became president.

In a bid to get back to business, Erdogan was on Monday to chair a cabinet and security meeting at his presidential palace that could extend the nine-month state of emergency brought in after the July 15 failed coup.

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.

- Reviving the death penalty? -

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.

German Foreign Minister Sigmar Gabriel said that if Ankara were to bring back the death penalty, the move would be "synonymous with the end of the European dream" and mark the end of decades of talks to enter the European Union.

In an interview in the Bild mass circulation daily to be published Tuesday, he warned Turkey that "joining would not work right now".

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