Erdogan
Turkish President Recep Tayyip Erdogan waves to supporters as he leaves Eyup Sultan mosque in Istanbul. Getty

Turkey's main opposition party demanded on Monday that a referendum granting President Tayyip Erdogan sweeping new powers be nullified after a narrow "Yes" vote that exposed bitter divisions and drew concern from European Union leaders.

Erdogan's supporters took to the streets to cheer, while opponents stayed indoors banging pots and pans in protest over the vote to bring the biggest overhaul in Turkish politics since the founding of the modern republic, abolishing the prime minister's post and concentrating power in the presidency.

Unofficial results showed a narrow victory with 51.4 percent of votes cast in favour. Official results are due in 12 days.

Erdogan, a populist with a background in once-banned Islamist parties, has ruled since 2003 with no real rival, while his country emerged as one of the fastest-growing industrial powers in both Europe and the Middle East.

He has also been at the centre of global affairs, commanding NATO's second-biggest military on the border of Middle East war zones, taking in millions of Syrian refugees and controlling their further flow into Europe.

Critics accuse him of steering Turkey towards one-man rule and exacerbating divisions that could increase instability. The two largest opposition parties both challenged Sunday's referendum, saying it was deeply flawed.

The pro-Kurdish opposition Peoples' Democratic Party said it presented complaints about unstamped ballots affecting 3 million voters, more than twice the margin of Erdogan's victory.

The main secularist opposition People's Republican Party said it was still unclear how many votes were affected.

"This is why the only decision that will end debate about the legitimacy (of the vote) and ease the people's legal concerns is the annulment of this election," deputy party chairman Bulent Tezcan said.

Tezcan said he would if necessary go to Turkey's constitutional court - one of the institutions that Erdogan would gain firm control over under the constitutional changes, through the appointment of its members.

Erdogan has long said the changes were needed to end chronic instability that plagued the country over decades when the military repeatedly tried to seize power from weak civilian governments.

"For the first time in the history of the Republic, we are changing our ruling system through civil politics," he said in a victory speech.

But the divisions revealed by the narrow referendum result could also herald more unrest to come. The changes won strong backing in conservative rural areas, but were just as strongly opposed in Istanbul and other cities, as well as in the restive Kurdish southeast.

"ERDOGAN'S RESPONSIBILITY"

The president survived a coup attempt last year and responded with a crackdown, jailing 47,000 people and sacking or suspending more than 120,000 from government jobs such as schoolteachers, soldiers, police, judges or other professionals.

The changes could keep him in power until 2029 or beyond, making him easily the most important figure in Turkish history since state founder Mustafa Kemal Ataturk built a modern nation from the ashes of the Ottoman empire after World War One.

Response from Europe, which has had increasingly strained ties with Turkey, was cautious, awaiting the judgment of international observers later on Monday. Germany, host to some 4 million Turks, said it was up to Erdogan himself to heal the rifts that the vote had exposed.

"The tight referendum result shows how deeply divided Turkish society is, and that means a big responsibility for the Turkish leadership and for President Erdogan personally," said Chancellor Angela Merkel and Foreign Minister Sigmar Gabriel in a joint statement.

In a signal of the direction he now plans for his nation, Erdogan said he could call a referendum to restore the death penalty. Such a move would effectively put an end to Turkey's decades-long bid to join the European Union, which has been the impetus for years of legal, social and economic reforms, and also an important part of how many Turks defined themselves.

Relations with Europe were strained during the referendum campaign when EU countries including Germany and the Netherlands barred Turkish ministers from holding rallies to support the changes. Erdogan provoked a stern German response by comparing those limits on campaigning to the actions of the Nazis.

CELEBRATION

Thousands of Erdogan supporters waved flags and blasted horns into the early hours on Monday in celebration of a man who they say has transformed the quality of life for millions of pious Turks marginalized for decades by the secular elite.

There were scattered protests against the result, but these were more sporadic. In some affluent, secular neighbourhoods, opponents stayed indoors, banging pots and pans, a sign of dissent that became widespread during anti-Erdogan protests in 2013, when the police crushed demonstrations against him.

The High Electoral Board (YSK) confirmed late on Sunday the results had shown the "Yes" campaign with 1.25 million more votes than the "No" camp. The official results are expected within 12 days.

The main opposition Republican People's Party (CHP) demanded a recount of up to 60 percent of the votes. It cited as a potential irregularity a last minute decision by the electoral board to count ballots that had not been stamped by officials.

Erdogan said 25 million people had supported the proposal, which will replace Turkey's parliamentary system with an all-powerful presidency. That was a smaller mandate than the decisive result for which he and his ruling AK Party had aggressively campaigned.

Nevertheless, by ending uncertainty the result triggered a two percent rally in the Turkish lira from its close last week. It traded at 3.6380 against the U.S. dollar early on Monday, firming from 3.7220 on Friday.

'NO EARLY ELECTIONS'

Under the changes, most of which will only come into effect after the next elections due in 2019, the president will appoint the cabinet and an undefined number of vice-presidents, and be able to select and remove senior civil servants without parliamentary approval.

There has been some speculation that Erdogan could call new elections so that his new powers could take effect right away. However, Deputy Prime Minister Mehmet Simsek told Reuters there was no such plan, and the elections would still be held in 2019.

"Yesterday the president made it very clear that elections will be held in November 2019," he said. "It is very clear. We have work to do."

Erdogan served as prime minister from 2003 until 2014, when rules were changed to hold direct elections for the office of president, previously a ceremonial role elected by parliament. Since becoming the first directly elected president, he has set about making the post more important, like the executive presidencies of France, Russia or the United States.

In a sign of his authority, he was set to chair a cabinet meeting later on Monday, a role traditionally carried out by the prime minister although he has chaired such meetings before.

Pro-government media painted the result as a victory for the Turkish people, transforming a constitution left over from a 1980 military coup. The Sabah daily hailed "The People's Revolution". The Star's headline was "The People's Victory".

However, the opposition daily Cumhuriyet's headline said "The ballot box is overshadowed", reporting opposition objections to what they said were irregularities in the voting.

(Reporting by Reuters bureau in Istanbul and Ankara; Writing by Gulsen Solaker, Daren Butler, David Dolan and Dominic Evans; Editing by Peter Graff)