Turkish President Recep Tayyip Erdogan on Sunday said the tightly-contested referendum on expanding the powers of the head of state was a vote for the future of Turkey.
The opposition Republican People's Party, however, planned to file an objection to the results, claiming they were manipulated.
Peter Altmaier, Merkel's chief of staff, said on German broadcaster ARD that the result showed there was a "very lively political debate" in Turkey, but it was necessary to wait for official results before commenting further.
The Yes vote gives Turkey's government the authority to replace the country's century-old parliamentary system with a presidential model.
Before Erdogan claimed victory, thousands of his supporters converged at the Ankara headquarters of the ruling Justice and Development Party, or AKP, which Ergodan founded.
Taking advantage of being an incumbent in gaining patriotism votes won President Recep Tayyip Erdogan what he had longed for. "But in Turkey, a country that has so many problems, in societies like ours, the image of strong leadership is necessary to command both fear and respect and trust in society".
Two opposition parties have complained of a number of irregularities in Sunday's vote, including an electoral board decision to accept as valid ballots that did not bear the official stamp. Hundreds of demonstrators marched in a central neighbourhood in Istanbul, clanking pots and pans and chanting "this is just the beginning, the struggle will continue'".
Erdogan has sought to ridicule CHP's Kemal Kilicdaroglu, describing him as ineffective and associating the "No" vote with support for terrorism.
According to results from Turkey's state-run Anadolu news agency, the "yes" vote won by 51.2 percent compared to 48.8 percent for the "no" camp with 100 percent of the votes tallied.
On Monday, many will be closely watching the initial assessment of the Organization for Security and Cooperation in Europe (OSCE), which had an observer mission in Turkey.
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The post of prime minister would be abolished after the 2019 national elections and time-limits on the newly-empowered presidency would be reset.
Erdogan survived a coup attempt last July, which he has blamed on his former ally and current nemesis Fethullah Gulen, an Islamic cleric living in the United States.
European bodies reacted with caution to the early results of the referendum.
Erdogan supporters say the changes are needed to establish stability in the country, which is facing unrest by Kurdish groups in eastern Turkey and a flood of refugees from the civil war in neighboring Syria.
"God willing, these results will be the beginning of a new era in our country", he said. "Israel, America and Germany are creating pressure over our country to take our money away from us", he said.
"Today, Turkey gave a historic decision on its governance system which has been an immemorial matter of debate for 200 years", Erdogan said, hailing the vote, which followed a two-month campaign.
Shortly after the vote, Erdogan repeated his intention to reinstate capital punishment in Turkey.
The pro-Kurdish People's Democratic Party (HDP) also claimed they had information that voter fraud was implicated in up to four per cent of the ballots.
"(The referendum result) will profoundly change the way the country is governed, because the system is moving from being a parliamentarian one to a presidential one, (which) critics would suggest is giving overdue power to the office of the presidency", Ahmet Kasim Han, Associate Professor, Kadir Has University, told CNN.
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