ANKARA: Turkey's prime minister has declared a victory based on unofficial results for backers of a referendum to greatly expand the powers of President Tayyip Erdogan.

Results carried by the state-run Anadolu news agency on Sunday showed the 'yes' vote had about 51.3 per cent compared to 48.7 per cent for the 'no' vote with nearly 99 per cent of the vote counted.

Addressing thousands of flag-waving supporters on Sunday night, Prime Minister Binali Yildirim said the 'unofficial' final result is 'yes" for the constitutional referendum.

Yildirim spoke on the balcony of his governing AK Party headquarters in Ankara, addressing a crowd of about 3000 people who waved flags and chanted the name of the country's president, Recep Tayyip Erdogan.

Foreign Minister Mevlut Cavusoglu told a group of supporters in his hometown of Antalya: 'As of now, there is a truly new Turkey. There will be stability and trust in the new Turkey.'

The changes doing away with Turkey's parliamentary system of government constitute one of the most radical political reforms since the Turkish republic was established in 1923.

The package of 18 amendments will abolish the office of prime minister and give the president the authority to draft the budget, declare a state of emergency and issue decrees overseeing ministries without parliamentary approval.

But Turkey's main opposition party says it will challenge 37 per cent of the ballot boxes counted in the referendum.

Republic People's Party, or CHP, Deputy Chairman Erdal Aksunger predicted that the figure could even increase to 60 per cent.

'Since this morning, we have determined some 2.5 million problematic votes,' he said.

The country's pro-Kurdish opposition party, which also opposed the constitutional changes, says it plans to object to two-thirds of the ballots.

Germany's foreign minister, meanwhile, said it was important to 'keep a cool head' after the Turkish referendum, whatever the outcome.

Sigmar Gabriel said in a brief statement on Sunday that it was a good thing the bitter campaign for the referendum is over.

Tensions flared between the governments of Turkey and European countries, particularly Germany and the Netherlands, over restrictions on Turkish politicians campaigning for the votes of compatriots who live abroad.

Erdogan supporters were celebrating with fireworks in Istanbul as the vote count was winding down.