Analysts said voters lost patience with both a cost-of-living crisis driven by near 70 per cent inflation and Erdogan's divisive political style.
The result bruised his hopes of adopting a new constitution, which could potentially extend his rule beyond 2028 when his term ends, they said. Though AKP and its allies have a majority in parliament, Erdogan would need broader support or a successful referendum for a new constitution.
Erdogan delivered a sombre and introspective speech in the early hours of Monday. "This is not an end for us, but actually a turning point," he said, acknowledging a "loss of altitude" for the AKP.
"If we made a mistake, we will fix it," he told crowds gathered at AKP headquarters in Ankara, without indicating what changes he might make within his party or in policy.
In response, Turkish stocks rose and the lira - which has shed more than 80 per cent of its value in five years - touched another record low versus the dollar on a holiday for many world financial markets.
INFLATION BITES
Erdogan made an abrupt U-turn in economic policy after his triumph in a national election last year, resulting in aggressive interest rate hikes to rein in inflation expectations that soared under his years-long unorthodox policy stance.
Erdogan has asked for patience with slower economic growth and high borrowing costs, promising reprieve later this year, and Finance Minister Mehmet Simsek said on Monday the belt-tightening programme would carry on.
But AKP election candidates were thumped in the cities of Istanbul and Ankara and even in deeply pro-Erdogan strongholds like Bursa, Afyonkarahisar and Adiyaman provinces.
"I think it's mainly about the economy and in particular the inflation...story. I think voters decided to punish Erdogan for these reasons," said Wolfango Piccoli, co-president of political risk consultancy Teneo.
He said AKP lost control of industrial regions where lots of workers are on a minimum wage, which has trailed inflation despite big rises.
AKP overall "suffered due to over-confidence hubris," Piccoli said, pointing to the success of the Islamist New Welfare Party, which emerged as the third biggest party in a big surprise, with 6.2% support.
New Welfare benefited by taking an even more hardline stance than Erdogan against Israel over the Gaza conflict, which helped draw pious voters away from the Islamist-rooted AKP, analysts said.
'SICK OF FIGHTING AND ARGUING'
The CHP - the party of modern Turkey's founder Mustafa Kemal Ataturk - won near 38% support nationwide, more than two points ahead of the AKP and shattering the ceiling of 25% support it has had this century.
Opposition Cumhuriyet newspaper called it "A historic victory," that taught Erdogan a lesson.
The CHP's Imamoglu won 51% support in Istanbul, Turkey's largest city, 11 percentage points ahead of his AKP challenger despite polls pointing to a close race.
He won despite the collapse of an opposition alliance after last year's election defeats, reaching out to Kurds and others typically outside of the secularist CHP base.
"The period of one-person rule has ended as of today," Imamoglu, 53, told thousands of jubilant supporters on Sunday evening.
The former businessman, who entered politics in 2008, had defeated Erdogan's candidate in the local election five years ago, ending 25 years of rule in the city by AKP and its Islamist predecessors. He is now touted as a presidential challenger.
"We didn't vote for (AKP) obviously due to the economic conditions and promises that were not kept," said accountant Onur Hizmetci, 42, adding he had voted AKP the last 15 years.
"All parties need to move away from polarisation and do something for our country with unity," he said in a public square on the Asian side of Istanbul. "People are sick of fighting and arguing."