Turkey's main opposition is headed for a decisive win in a re-run of the election for Istanbul mayor, in a major setback for President Recep Tayyip Erdogan.
Ekrem Imamoglu, mayoral candidate of the secularist Republican People's Party (CHP), was leading with 54 percent of votes versus 45 percent for Mr Erdogan's AK Party (AKP) candidate, with more than 99 percent of ballots opened, Turkish broadcasters said.
"Today, 16 million Istanbul residents have renewed our faith in democracy and refreshed our trust in justice," Mr Imamoglu told supporters.
His AKP opponent, former prime minister Binali Yildirim, congratulated him and wished him "all the luck" in serving Istanbul, Turkey's commercial hub. Mr Erdogan also tweeted his congratulations to the CHP candidate.
Mr Imamoglu won the original mayoral election on 31 March by a narrow margin that prompted the Islamist-rooted AKP to demand a re-run, citing what it said were voting irregularities.
The High Election Board's decision to grant that request drew sharp criticism from Turkey's Western allies and from Mr Erdogan's opponents at home, stirring concerns about the rule of law and raising the stakes in a re-run that many Turks saw as a test of their country's democracy.
Broadcasters put the CHP's lead on Sunday at about 700,000 votes, eclipsing the roughly 13,000-vote margin in March.
The election board said it would announce the election results as soon as possible.
Mr Imamoglu, a former businessman and district mayor who waged an inclusive campaign and avoided criticising Mr Erdogan, said he was ready to work with the AKP to tackle Istanbul's problems, including its transport gridlock and the needs of its Syrian refugees.
"In this new page in Istanbul, there will from now on be justice, equality, love, tolerance; while misspending (of public funds), pomp, arrogance and the alienation of the other will end," he said.
The handover of power in the mayor's office could shed further light on what Mr Imamoglu said was the misspending of billions of lira at the Istanbul municipality, which has a budget of around $US4 billion.
If confirmed, this second defeat in Istanbul would be a major embarrassment for the president and could also weaken what until recently seemed to be his iron grip on power. He had campaigned hard in Istanbul and had targeted Mr Imamoglu directly with accusations of lying and cheating.
Turkey's economy is now in recession and the United States, its NATO ally, has threatened sanctions if Mr Erdogan goes ahead with plans to install Russian missile defences.
The uncertainty over the fate of Istanbul and potential delays in broader economic reforms have kept financial markets on edge. Turkey's lira currency tumbled after the decision to annul the March vote and is down nearly 10 percent this year in part on election jitters.