Polling stations closed across Turkey on Sunday for municipal elections, election board chair Ahmet Yener told reporters in Ankara.
Preliminary results are expected later this evening.
The race for control of the largest city Istanbul and other industrial hubs will test President Recep Tayyip Erdoğan's popularity amid a cost-of-living crisis and 10 months after he extended his rule with a third term.
Roughly 61 million people were eligible to vote, including around 1 million first-time voters.
Observers pointed to an unfair election campaign with 70-year-old Erdoğan enjoying far larger state resources and controlling 90% of the mainstream media.
Istanbul will see one of the key clashes in the election.
The candidate for the secular Republican People's Party (CHP) in Istanbul, current Mayor Ekrem Imamoğlu, is seeking election for another term.
Imamoğlu wrestled control from Erdoğan's ruling AKP in Istanbul in 2019, ending a 25-year stint by Islamic conservatives.
A second Imamoğlu victory would set him up as the main rival to Erdoğan in the next presidential elections, due in 2028.
Polls showed a close race between Imamoğlu and Erdoğan ally Murat Kurum, the former urban planning minister from Erdoğan's AKP.
Istanbul, a metropolis of 16 million, holds a special status for Erdoğan, whose political rise started there.
When his Istanbul candidate lost by a small margin in 2019 local elections, Erdoğan's government quickly moved to cancel the vote.
In repeat polls, Imamoğlu won by an even larger margin, handing Erdoğan his worst political loss.
Observers have warned of the risk of a further slide into “authoritarianism” if Istanbul, with its around $16 billion annual budget, is returned to AKP control.
“In an increasingly authoritarian Turkey that concentrates resources in the government, opposition parties need access to municipal resources in order to survive,” said political analyst Berk Esen.
Imamoğlu also faces a potential political ban in a court case which critics say is politically motivated.
“Imamoğlu is the opposition’s best candidate,” analyst Esen told dpa. "After another win, it would be very difficult to write Imamoğlu off... Erdoğan knows that too.”
Meanwhile, the atmosphere was tense in the pre-dominantly Kurdish south-east where Erdoğan's government replaced many pro-Kurdish politicians with appointed governors due to alleged terrorism ties in the 2019 polls.
At least two people died and 15 were injured after violence erupted, including firearms, over the mayoral races in south-eastern Diyarbakir and Siirt provinces, state news agency Anadolu reported.
The pro-Kurdish DEM party, which enjoys widespread support in south-eastern Turkey, said it was investigating the violence.
The party, under the name HDP at the time, secured 65 mayoral posts in the 2019 elections, but the central government in Ankara removed many of them from office.
Erdoğan accuses the party of links to the outlawed Kurdistan Workers' Party (PKK), allegations the DEM denies.