By Daren Butler and Birsen Altayli

ISTANBUL (Reuters) - Turkish authorities are investigating an executive at Dogan Holding, the conglomerate said on Thursday, heightening concern President Tayyip Erdogan's crackdown after a failed coup may now spread to the country's top companies.

Since a attempted putsch in July, authorities have detained or dismissed more 125,000 people in the police, judiciary and civil service and arrested 36,000. Nearly 600 companies have been seized, many of them smaller provincial firms.

Companies with longstanding ties to the exiled Muslim cleric Fethullah Gulen, whom Erdogan has accused of masterminding the failed coup, have also been targeted in the crackdown.

Until now, Turkey's leading firms, mostly Istanbul-based and affiliated with the secular elite, have been largely untouched.

But on Thursday, Erdogan announced an acceleration of the campaign to root out supporters of what it calls the "Gulenist Terror Organisation" or "FETO" and members of the outlawed Kurdistan Workers Party (PKK).

"A serious cleansing in public institutions, media and business world is underway within the frame of struggle against FETO and the PKK," he said in a speech in Ankara.

Dogan, which has interests in media, finance, energy and tourism, was founded by Aydin Dogan, a prominent figure in the secular establishment. He has previously clashed with Erdogan and his group has faced multibillion-dollar tax fines.

Dogan Holding said in a statement that its Ankara representative was under investigation. While the company itself has not been formally accused of anything, it also disavowed links to the U.S.-based Gulen.

The cleric has lived in self-imposed exile since 1999, denied the charges against him and condemned the coup.

State-run Anadolu Agency said the Dogan executive had been detained on the orders of the Istanbul prosecutor's office. An official at the prosecutor's office declined to comment.

RARE REBUKE

Shares in Dogan fell by 7 percent, while group company Hurriyet, which publishes a leading Turkish newspaper of the same name, fell 8.8 percent, becoming the biggest percentage decliner on Istanbul's BIST 100 index.

The overall stock market fell 2 percent.

In a rare public rebuke of government policy, the head of Turkey's biggest business lobby called for a swift end to the emergency rule instituted after the coup.

"Some emergency rule practices due to security concerns are resulting in a loss of confidence in the economy," Cansen Basaran-Symes said in a speech on Thursday. Her organization, TUSIAD, is seen as representing the Istanbul-based old-money elite.

By contrast, Erdogan, a pious Muslim, has encouraged the rise of a new breed of religious businessmen, many of them from provincial cities. Thanks in part to their dramatic success, and Erdogan's successful stewardship of the economy for more than a decade, Turkey was once seen as one of the world's most promising emerging markets.

But its star has since fallen. In September, Moody's cut its debt rating to "junk" status, citing worries about the rule of law. In November the lira currency suffered its worst monthly performance since the 2008 financial crisis.

Separately, Turkey's Garanti Securities said on Thursday that it was appointed to advise on the sale of 211 businesses seized by the state in the crackdown on companies suspected of having links to Gulen.

(Additional reporting by Behiye Selin Taner and Ece Toksabay; Writing by David Dolan; Editing by David Goodman and Tom Heneghan)