Istanbul, Turkey (CNN) Turkish President Recep Tayyip Erdogan has emerged victorious in a high-stakes election, seeing off the most serious challenge yet to his political dominance and tightening his grip on the nation he has ruled for 15 years.

Defeating the twin threats of a reinvigorated opposition and a weakened currency, Erdogan declared himself the winner even before official results were confirmed.

With 97.7% of the votes counted, Sadi Guven, chief of Turkey's Supreme Election Board, said Erdogan had won an absolute majority, avoiding a runoff against his principal challenger, Muharrem Ince. State media put Erdogan on 52.5%, well ahead of Ince on 31%.

"The winners of the June 24 elections are Turkey, the Turkish nation, sufferers of our region and all oppressed (people) in the world," Erdogan said in a victory address from a balcony at the AKP's offices in the capital Ankara in the early hours of Monday morning.

The results were a blow to Ince and his Republican People's Party, known as the CHP, who ran a spirited campaign that threatened to force Erdogan into a damaging runoff and deny his party control of parliament.

Conceding defeat, Ince warned that constitutional changes ushered in by Erdogan earlier in the year represented a threat to the country's democracy.

"A single person is becoming the head of the legislature, the executive, and the judiciary and this is a concern for a threat to the survival of the country," he told journalists. "Turkey has departed from democratic values and Turkey has broken its ties with the parliamentary system which it had."

"We're now in a one-man rule -- there's no mechanism to prevent arbitrary rule. We continue to have great concerns about this situation."

Expanded powers

Erdogan starts a new five-year term as president with sweeping new powers granted in a narrowly won referendum last year, denounced by his critics as a blatant power grab. Under the new system, the office of prime minister is abolished, parliament's powers curtailed and the president is accorded wide-ranging executive authority.

Turkish Foreign Minister Mevlut Cavusoglu said Monday that the implementation of the constitutional amendments "is important for our stability and economic development."

"It's a new system for us," he said, adding that it was approved by the Turkish electorate.

In any case, Erdogan's Justice and Development Party (AKP) and its allied Nationalist Movement Party's (MHP) retained their parliamentary majority.

"Turkey has decided to take the side of growth, development, investment, enrichment and a reputable, honorable and influential country in all areas in the world," Erdogan told cheering supporters.

Erdogan had earlier claimed victory in Istanbul, Turkey's largest city. "I would like to congratulate our nation once again. This has been another test of democracy and we have passed this test successfully," he said.

Supporters of Turkey's ruling Justice and Development Party, or AKP, leader, President Recep Tayyip Erdogan celebrate outside the party headquarters.

Russia congratulated Erdogan on his re-election, with a government statement saying President Vladimir Putin "stressed that the outcome of the vote fully confirms Erdogan's great political authority, broad support of the course pursued under his leadership towards solving vital social and economic tasks facing Turkey, and enhancing the country's foreign policy positions."

Cheering supporters

After a day in which millions cast their votes, throngs of Erodogan supporters waited until the early hours of Monday morning to hear Erdogan speak. In the largest city, Istanbul, cars in the street and boats on the Bosporus honked their horns and fireworks lit up the sky.

Erdogan supporter Emine Kilic said that she was happy with the result, despite what she sees as virulent opposition to the ruling party.

"I think the election outcome is very very good but it could have been much better," she said from Istanbul. "That's because of the evil forces and many traitors inside the country. But still it's good we won!"

Others decried the result, fearing that the political climate was becoming more intolerant.

"I am very, very upset," Burcu, an Istanbul resident, said. "Really, really... as a young woman... I am only 25. I want to live in this country but as a young woman... it is becoming harder to live in this country, women are not respected and the education system is upside down."

Around 59 million people were eligible to vote in both presidential and parliamentary elections Sunday . Erdogan said the participation rate was 90%, an extraordinarily high number for any election.

President Recep Tayyip Erdogan casts his vote in Istanbul on Sunday.

Cavusoglu, the foreign minister, lauded the turnout. "Turkish society is very engaged with democracy. If you look at the turnout rates its almost 90%. Under the leadership of our president, the ruling party will be working further to strengthen democracy."

Earlier, state news agency Anadolu reported that the President had sailed through the snap vote at 52.7%, with more than 96% of the ballots counted. It also said the AKP-MHP coalition had more than 53% of the parliamentary vote, with over 98% of the votes counted.

But the opposition CHP had said that around half the ballot boxes had not yet been counted, and called on party monitors to stay by the ballot boxes and keep watching.

People shout slogans and let off flares after counting of votes in Turkey's presidential and parliamentary elections commenced.

A spokesman for Erodgan's AKP, Mahir Unal, dismissed the accusations and warned party leaders of "harsh outcomes" to any provocations.

Opposition parties will have five days after the vote to lodge any complaints or challenges.

Candidate Muharrem Ince votes in the northwestern city of Yalova on Sunday.

Erdogan has dominated Turkish politics since his rise as prime minister in 2003 and has transformed the nation. He implemented policies that encouraged sustained economic growth and development, he challenged Turkey's secular foundations by bringing Islamic conservatism to public life and he gutted public institutions by having tens of thousands of people -- many of them his critics -- arrested after a failed military coup in 2016.

An elderly woman in a wheelchair leaving a polling station in Istanbul on Sunday.

Erdogan himself called the snap elections 18 months early, as he faces battles on several fronts: Turkish voters are feeling the pain of soaring inflation, a plunging currency and high interest rates as the economy falters, and the normally splintered opposition is largely united against him for the first time in years.

By offering a wider than usual range of presidential candidates, the opposition had hoped to split the vote enough ways to leave the front-runner with less than 50% of the ballots.

Erdogan has won several consecutive elections to become Turkey's longest-serving leader, but a strong campaign by Ince triggered speculation the President might fail to win outright.