17/04/2017 3:52 AM AEST | Updated 17/04/2017 4:03 AM AEST

Turkey Referendum: President Erdogan Declares Victory, But Opponents Contest

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A supporter of the 'yes' brandishes a picture of Turkish president Recep Tayyip Erdogan among other supporters waving Turkish national flags during a rally on Sunday

Turkish President Recep Tayyip Erdogan is thanking allies and supporters for the passage of a constitutional referendum he backed that greatly expands his powers, officials said.

Erdogan called nationalist allies on Sunday night to congratulate them for the result as vote-counting was continuing, but nearly completed, the Associated Press reported.

With 97 percent of votes counted Sunday, 51.4 percent backed the constitutional changes.

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

Erdogan says he is “grateful” to the people who “reflected their will’

However, Turkey’s main opposition party says it will challenge 37 percent of the ballot boxes counted in Turkey’s referendum to increase the president’s powers.

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

Aksunger said: “Since this morning, we have determined some 2.5 million problematic votes.”

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

The Peoples’ Democratic Party said on its Twitter account: “Our data indicates a manipulation in the range of 3 to 4 percent.”

Germany’s foreign minister says it will be important to “keep a cool head” after the Turkish referendum on increasing the president’s powers, whatever the outcome.

Foreign Minister Sigmar Gabriel said in a brief statement Sunday that it’s 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.

Gabriel says a close result, as expected, appears likely.

He said: “However the Turkish people’s vote goes in the end, we are well-advised to keep a cool head and proceed with calm.”