By Huseyin Hayatsever and Jonathan Spicer ANKARA (Reuters) -Tayyip Erdogan's main political opponents have faced an unprecedented crackdown that has seen more than 500 detained in just nine months, according to a Reuters review of a sprawling investigation that has accelerated dramatically in recent days. Turkey's president says the probe tackles what he calls a corrupt network that is like "an octopus whose arms stretch to other parts of Turkey and abroad." The investigation, which began in Istanbul but has spread across the country, has targetted only municipalities run by the main opposition Republican People's Party, or CHP, the party of modern Turkey's secularist founder Mustafa Kemal Ataturk. The CHP denies the corruption allegations and calls them a naked attempt to eliminate a democratic alternative for Turks, a charge the government refutes. The crackdown tightens Erdogan's two-decade grip on power at a time that Turkey's influence in the Middle East and Europe has grown. For this reason, diplomats and analysts say, it has garnered only muted criticism from Western allies as a threat to democracy even as street protests erupted in the spring. According to the review of legal filings and state disclosures, 14 elected CHP mayors, including Istanbul's Ekrem Imamoglu - Erdogan's main rival - and more than 200 party members or local officials have been jailed pending trial. Not since a series of coups in the 1960s, 1970s and 1980s have such high-profile political leaders been removed from office on the basis of as yet unpublished evidence, which suspects' lawyers dismiss as fabricated. "These investigations are being used as a tool for political attrition rather than objective investigation of concrete events," said Ertugrul Gunay, a former culture and tourism minister in Erdogan's cabinets between 2007 and 2013. He resigned from the ruling AK Party (AKP) after thousands of Turks were arrested over the anti-government Gezi Park protests of 2013. The latest legal drive, though smaller in scale, has gone further in targetting a would-be future government, riding high in the polls. It reflects "anxiety and panic that (Erdogan's) ruling party has for the next elections," Gunay told Reuters. Erdogan and his ministers have repeatedly rejected as unfounded critics' accusations of judicial interference, saying the independent courts need time to sort through evidence. They say such criticism reflects an opposition party reckoning with its illegal practices and internal strife, and undermines public trust. "This is a legal process, not a political one. We are not involved in any aspect of this process," Erdogan told his AKP MPs in parliament on Wednesday. MORE THAN 220 IMPRISONED At the centre of the investigation is Imamoglu, mayor of Istanbul's 17 million people, who was jailed in March pending a court hearing on corruption charges he denies. He is the CHP presidential candidate in any future election, and his arrest sparked the biggest protests since Gezi and a sharp lira selloff, both of which have since abated. But beyond Imamoglu - who from behind bars still leads Erdogan in some polls - the Reuters review found that more than 500 people were detained and questioned since the probe began in October last year, including at least 202 since last week alone. Of those, more than 220 were imprisoned or put under house arrest, according to the review, which was based in part on a compilation of reports by state-run Anadolu Agency. Erdogan's office and the Justice Ministry did not respond this week to a detailed request for a tally of detentions and arrests, and for a comment on the Reuters review's findings. Reuters further found that at least 36 people, mostly those in the private sector doing business with municipalities, provided a second statement to prosecutors from prison under the "effective repentance" provision of Turkish law - after which 32 of them were released from prison under judicial control measures. These statements have identified more suspects, disclosures from prosecutors and others show. Since Tuesday last week the investigation has spread to Izmir, Turkey's third-largest city, as well as Antalya, Adana and Adiyaman - all won by the centrist CHP over Erdogan's ruling conservative AKP in last year's March municipal elections, the party's biggest ever electoral defeat. TENTACLES Erdogan has in recent months predicted, correctly, that more charges and detentions were to come, further stoking concerns over political interference. Days after his octopus comment in May, five district mayors from Istanbul and Adana were arrested on corruption charges. Erdogan's office and the Justice Ministry did not respond to a request for comment on critics' claims that authorities' public comments about the probe harm judicial independence, and that it unfairly targets only the CHP for political gain. Turkey's Directorate of Communications released a list of past AKP mayors who were convicted on similar charges in separate probes, saying claims that the CHP is exclusively targetted are "entirely unfounded". Most of those listed were investigated after leaving office and were not jailed pending trial. The Reuters review of the latest probe shows no legal actions were taken in the 14 of Istanbul's 39 districts run by the AKP. Mehmet Pehlivan, Imamoglu's lawyer who was also jailed last month on criminal-membership charges he denies, told Reuters from prison that the investigation seeks for the first time to criminalise the right to practice law and to a legal defence. He said the mayor faces "not a single concrete piece of evidence". Prosecutors have not yet issued indictments. In one window into the probe, a 121-page transcript of police questioning seen by Reuters shows that Imamoglu faced one claim that he colluded with a group of men who allegedly met at a cafe to discuss bribe payments. Police asked how his phone connected to the same cellular tower as those of the men at least 150 times, the documents show. He responded that his home at the time was close to the cafe so his phone would naturally use the same tower. A spokesman for the prosecutor did not immediately comment on the police questioning. The CHP has rejected all corruption allegations against its municipal officials but has said it will investigate after Turkey's TRT state broadcaster released footage of the deputy mayor of Manavgat, in the southern Antalya province, allegedly accepting bribes. FUTURE ELECTION Turkey has in the past seen waves of mass arrests of pro-Kurdish leaders, civil society members, military officers and outlawed groups, especially during Erdogan's increasingly authoritarian tenure. The CHP had been relatively spared in the Erdogan era, in which it lost a string of elections to his AKP since 2002. Though the next presidential vote is not scheduled until 2028, it will need to come sooner if Erdogan wants to run again. He could also seek to amend the constitutional two-term limit. The CHP mayors, including 14 in jail and one under house arrest, all deny the corruption, bribery and terrorism-related charges for which they await a court decision. Some have been suspended from duty. CHP Chairman Ozgur Ozel said the mayors "have fallen prisoner to this coup against Ataturk's party" in a speech on Sunday, that itself prompted a separate probe on charges including insulting the president. (Reporting by Huseyin Hayatsever and Jonathan Spicer; Additional reporting by Ece Toksabay in Ankara and David Gauthier-Villars in Istanbul; Editing by Alexandra Hudson)