While Sunday's local elections carry less importance after Turkey's assumption of an executive presidential system in 2017, they are widely seen as the most important test of President Recep Tayyip Erdogan's presidency yet.
The turnout was a very high 84.52 percent nationwide, according to the state-owned Anadolu news agency.
Erdogan, who was elected a year ago as the country's first executive president, said the next polls would be held in June 2023, adding that Turkey would carefully implement a "strong economic programme" without compromising on free-market rules.
The mayor's seat in the city of 15 million is hugely important.
"Today's elections are as historic as that of 1994", journalist Rusen Cakir tweeted.
Following his party's defeat in several municipalities, Erdogan pledged to focus on the economy in the run-up to national elections in 2023, noting that there's "a lot of work ahead".
The CHP took over the capital after 25 years, which was governed by the ruling party AKP since it came to the rule.
Even if the discussion over who won Istanbul might continue for another day or so, Ankara, Antalya, Adana, Mersin and many other cities that went to the main opposition Republican People's Party (CHP) and the İYİ (Good) Party opposition alliance underline a remarkable demand for change.
Trump adds a deadline on his threat to close the border
A US Border Patrol Agent rides a vehicle on the beach in San Diego seen through the border wall from Tijuana, Mexico. Asked if he would close the Mexican border to all trade , Mr Trump told reporters "It could be to all trade ".
Naturally, neither the victor nor the loser can accept the election result as an earnest reflection of the will of the people.
Supporters of opposition Republican People's Party (CHP) celebrate after early results of the local election in Ankara.
After campaigning nearly yearly for national elections, referendums and local votes, Erdogan's horizons have reduced to relatively short-term goals, Jenkins said, casting doubt on his stated plan to avoid bringing forward elections slated for 2023.
"There are 31,136 ballot boxes [in Istanbul]".
Pro-Kurdish People's Democratic Party (HDP) in its election campaign did not run candidates in some crucial provinces in the west, including Istanbul, as a tactic to weaken votes of the ruling party, which in its nature provided indirect support to the opposition's Nation's Alliance.
The opposition is also ahead in the contest for mayor of the largest city, Istanbul, the election commission says. The opposition won Ankara, a ruling party stronghold for decades, and was leading a tight race for mayor in Istanbul, according to unofficial figures Monday.
"This will certainly lead to an emerging new political landscape in Turkey", he said.
In Istanbul, AKP candidate and former prime minister Binali Yildirim had a narrowing lead of 0.14 percentage point over the main opposition party after 98 per cent of the ballots were opened.