* Raids staged two months before snap election
* Erdogan battling Gulen since 2013 graft probe
* Shares in Koza Ipek group, other media tumble (Adds group chairman's comments, British journalists)
By Daren Butler and Ayla Jean Yackley
ISTANBUL, Sept 1 (Reuters) - Turkish police on Tuesday raided the offices of a conglomerate linked to a U.S.-based cleric accused of plotting against President Tayyip Erdogan, a move seen by critics as a bid to silence opposition media two months ahead of an election.
Erdogan, who wants the ruling AK Party to win back a majority in a snap Nov. 1 election, accuses Gulen of seeking to overthrow him by means of a "parallel structure" of supporters in the judiciary, police, the media and other institutions.
Police searched the offices of 23 companies in the Koza Ipek group, whose interests range from mining to TV stations, on suspicion of providing financial support to the "Gulenist Terrorist Group", the state-run Anadolu Agency said.
It said six people had been detained.
There was no immediate comment from the police. Interior ministry officials declined comment.
Erkan Akkus, news editor at Kanalturk and Bugun TV which are part of Koza Ipek's media business, said the holding company's headquarters and the chairman's home were being searched.
"The aim here is to silence the opposition media ahead of an election," Akkus told Reuters.
"It is wrong to see this as aimed just at our group. They are starting with us to test the waters, and if it doesn't spark an outcry, it could then spread to other media groups."
Koza Ipek Chairman Akin Ipek denied any wrongdoing in a telephone interview with the group's TV stations and said all documents sought by financial crimes police had been delivered.
"If they find a single illegal penny I will give them the company as a present," he said.
"It is a total fantasy, an empty slander."
Shares in Koza Ipek companies, including energy firm Ipek Dogal Enerji and miner Koza Madencilik slumped around 15 percent. Shares in other media firms not affiliated with Gulen also declined amid concern that the searches would widen.
Anadolu Agency said the raids did not target the group's media companies. Bugun newspaper editor Erhan Basyurt said media firms appeared on the list of companies covered by the search warrant but that their premises had not yet been searched.
Opposition party leaders voiced concern about the police raids, while Turkey's new European Union Minister Ali Haydar Konca - a member of the pro-Kurdish opposition who joined an interim power-sharing cabinet last week - warned against any raids targeting the media.
"I am worried that operations targeting the media will create great concern across the world about whether Turkey is a democratic country," he told a news conference.
As a protest against what it said was increasing government pressure on critical media, the Sozcu newspaper - one of Turkey's highest-circulating dailies - carried no news on its front page and left its editorial columns blank.
"If Sozcu is silent, Turkey is silent," it said in a banner headline, saying 57 court cases had been opened against the paper in the last year.
Erdogan called the snap poll for Nov. 1 after the Islamist-rooted AK Party he founded and which has governed Turkey for more than a decade failed to forge a coalition with rival parties following an inconclusive June election.
Tuesday's operation came after a whistleblower account on Twitter named "Fuat Avni", which has correctly forecast previous police raids, said last week that Erdogan planned a crackdown on media critical of the government ahead of the election.
Last week mainstream daily Milliyet dismissed five prominent journalists.
On Monday, a court charged two British journalists and their translator working for Vice News with having links to a terrorist organisation after they were detained while reporting from the mainly Kurdish southeast.
Late last year, police detained dozens of people in raids on media outlets with ties to Gulen. Some of those journalists have been charged with terrorism-related crimes.
The operation against the Koza-Ipek group added to the worries of investors already rattled by political uncertainty and a surge in fighting between Turkish security forces and Kurdish militants.
"There is a risk that this sort of thing can intensify into the election and it really is underscoring the weak investment climate within Turkey," said Manik Narain, EM strategist at UBS.
The cleric Gulen, who has lived in self-imposed exile in Pennsylvania since 1999, has denied in the past any ambition to overthrow Erdogan. Gulen has also been charged for alleged "terrorist" activities.
The battle between Erdogan and Gulen became public in December 2013 when a corruption investigation targeting Erdogan's inner circle came to light. Related court cases were subsequently dismissed following a large-scale reorganisation within the police and judiciary.
Erdogan, who was then prime minister, blamed the cleric's supporters for the corruption allegations and purged thousands of police and members of the judiciary he deemed loyal to Gulen.
In his early years in power, Erdogan drew on Gulen's influence in the judiciary to help tame the army - which had toppled four governments since 1960, including Turkey's first Islamist-led cabinet - through a series of coup plot trials. (Additional reporting by Gulsen Solaker in Ankara and Sujata Rao in London; Writing by Daren Butler; Editing by Nick Tattersall and Gareth Jones)