ISTANBUL: Turkish President Recep Tayyip Erdogan on Wednesday asked forgiveness over his past alliance with US-based preacher Fethullah Gulen whom Ankara blames for last month's attempted coup.

In a rare show of public humility, Erdogan said he had failed to see the "true face" of Gulen, who cooperated closely with the Turkish strongman while he was mayor of Istanbul in the 1990s and after his Justice and Development Party (AKP) came to power in 2002.

Gulen, who has lived in exile since 1999 after charges were laid against him by Turkey's then secular authorities, has vehemently denied from his Pennsylvania compound all accusations of involvement in the coup.

"Despite everything, I feel sad that I failed to reveal the true face of this traitor organisation long before," Erdogan said in a televised speech, referring to Gulen's network which Turkey alleges is a terror group.

"I know that we have to give account both to our God and to our people. Let my God and my people forgive us," he said.

Gulen's influence in Turkey goes back to the premierships of Bulent Ecevit in the 1970s and he was a strong presence in Turkish politics before anyone had even heard of Erdogan.

As a new and untested forces in politics, the AKP was happy to team up with Gulen, sharing a conviction that religion should lie at the heart of politics.

But friction came to the fore in 2013 when a vast corruption scandal struck at the heart of the government, as well as Erdogan and his inner circle, that the authorities blamed on Gulen.

"I personally helped this structure despite differences of opinion on many issues due to a belief that we can agree at least on common denominators," Erdogan said.

The Turkish strongman has accused Gulen of running "the biggest theft ring in history which hijacked the past and future of tens of thousands of people". With the government implementing a relentless crackdown in the wake of the coup, Erdogan vowed the harshest consequences for anyone who even supported Gulen.

"From now on, anyone who lends an ear to the raving of that charlatan — that terrorist chief in Pennsylvania — has accepted what's coming to them."

Tens of thousands of people have been suspended from their jobs and more than 18,000 detained.

"From this moment, the era of suspicion is over and the era of fighting has started," he said.