U.S. President Donald Trump called Turkish President Tayyip Erdogan to congratulate him on his referendum victory and to thank him for supporting a U.S. missile attack on Syria in response to a chemical attack by Syrian government forces on April 4, the White House said in a statement.

Trump and Erdogan also agreed on the importance of holding Syrian President Bashar Assad accountable for the chemical attack and discussed the campaign against the Islamic State, the statement said.

Turkey voted on Sunday to switch to a presidential system, greatly increasing Erdogan's powers. Unofficial results, which the opposition said it would challenge, showed a narrow victory for him with 51.4 percent of votes cast in favour.

Earlier that day, a defiant Turkish President Tayyip Erdogan denounced the West's "crusader mentality" after European monitors criticized a referendum to grant him sweeping new powers, won with a narrow victory laying bare the nation's divisions.

Open gallery view Supporters of 'No' protest the results of a nationwide referendum that will determine Turkey's future, Istanbul, April 16, 2017. Credit: OZAN KOSE/AFP

Addressing a crowd of flag-waving supporters from the steps of his palace in Ankara, Erdogan told election observers to "talk to the hand" and said it would not be so important to Turkey if the European Union broke off accession talks.

Sunday's vote ended all debate on forging a stronger presidency, Erdogan said, vowing that implementation of the reforms would begin straight away. But the main opposition party rejected the result and called for the vote to be annulled.

Thousands of people marched through at least three neighborhoods of Istanbul, some chanting "Thief, Erdogan," "no to the presidency" and "this is just the beginning" after calls on social media for protests in several cities.

Election authorities said preliminary results showed 51.4 percent of voters had backed the biggest overhaul of Turkish politics since the founding of the modern republic.

Erdogan said concentrating power in the hands of the president is vital to prevent instability. But the narrowness of his victory could have the opposite effect: adding to volatility in a country that has lately survived an attempted coup, attacks by Islamists, a Kurdish insurgency, civil unrest and war across its Syrian border.

The result laid bare the deep divide between the urban middle classes who see their future as part of a European mainstream, and the pious rural poor who favor Erdogan's strong hand. Erdogan reiterated his readiness to restore the death penalty at several appearances on Monday, which would effectively end Turkey's decades-long quest to join the EU.

Open gallery view A woman waits for the arrival of Turkish President Tayyip Erdogan following the results of the referendum, Ankara, Turkey, April 17, 2017. Credit: STRINGER/REUTERS

"The crusader mentality in the West and its servants at home have attacked us," he told a crowd as he arrived at Ankara airport, in response to the monitors' assessment.

"We neither see, hear, nor acknowledge the political reports you'll prepare," he said later at the palace. "We'll continue on our path. Talk to the hand. This country has carried out the most democratic elections, not seen anywhere in the West."

The mission of observers from the 47-member Council of Europe, the continent's leading human rights body, said the referendum was an uneven contest. Support for "Yes" dominated campaign coverage, and the arrests of journalists and closure of media outlets silenced other views, the monitors said.

"In general, the referendum did not live up to Council of Europe standards. The legal framework was inadequate for the holding of a genuinely democratic process," said Cezar Florin Preda, head of the delegation.

While the monitors had no information of actual fraud, a last-minute decision by electoral authorities to allow unstamped ballots to be counted undermined an important safeguard and contradicted electoral law, they said.

Turkey's foreign ministry dismissed the observers' criticism as lacking objectivity and impartiality.

Open gallery view Supporters of 'No' protest the results of a nationwide referendum that will determine Turkey's future, Istanbul, April 16, 2017. Credit: YASIN AKGUL/AFP

The U.S. State Department said it had taken note of the European monitors' concerns and looked forward to a final report, urging the Turkish government to protect the rights and freedoms of all citizens, however they voted.

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.

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

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.