* Justice minister dismisses prosecutor as showman
* Erdogan's aides say his allegations are lies and slander
* Poll shows support for Erdogan's party slipping
By Daren Butler
ISTANBUL, Jan 9 (Reuters) - Turkey's government derided a high-profile prosecutor behind a damaging corruption investigation as a showman on Thursday, after he accused Prime Minister Tayyip Erdogan of threatening him and telling him to halt the inquiry.
An opinion poll, the first since the corruption scandal broke last month, showed Erdogan's AK Party losing some support, but still well ahead of any rivals.
Former deputy chief Istanbul prosecutor Zekeriya Oz, who was reassigned earlier this week as part of a government crackdown on judiciary and police, said his police protection vehicle, provided when he faced death threats during a coup plot investigation, had been removed without explanation.
"Those who carried out this unlawful action will be responsible for anything that happens to me personally or my family," he said in a statement.
Oz said he had been warned by two senior members of the judiciary to stop the corruption investigation, which has gripped Turkey for weeks and poses the biggest threat yet to Erdogan's 11-year rule.
"(They) told me the prime minister was angry at me ... They said the investigations against the government should be halted immediately or I would suffer harm," Oz said.
"I said to them the worst thing that could happen to me would be death and that, if I died, I would be a martyr in the line of duty and this would be an honour for me."
Erdogan's aides denied the prime minister had sent anyone to speak to Oz, describing his comments as lies.
"Prosecutors should conduct their duties within the framework of the constitution and the law," Justice Minister Bekir Bozdag told reporters. "If instead they put on a show, they end up weakening respect for them, trust in the law and belief in justice."
Oz is a well-known figure in Turkey, once considered a hero by the government for leading the 2007 "Ergenekon" investigations into alleged plots to overthrow Erdogan - trials which led to the conviction of more than 250 military officers, politicians, academics and journalists.
Erdogan's Islamist-rooted AK Party is widely held to have relied heavily on U.S.-based Islamic cleric Fethullah Gulen's influence in the police and judiciary in the Ergenekon trials, which helped break the power of an army that carried out three coups between 1960 and 1980 and forced an Islamist-led government from power in 1997.
But the erstwhile allies now stand bitterly divided. Erdogan's followers view the corruption scandal as a "dirty plot" by Gulen's secretive Hizmet (Service) movement esconced within the judiciary and police to undermine him and thwart his ambition to run for the presidency in August elections.
Each side accuses the other of manipulating police and compromising the independence of the judiciary.
The government has ousted hundreds of police since the graft affair erupted on Dec. 17 with the detention of dozens of people including businessmen close to the government and three cabinet ministers' sons. Among the dozens questioned, most have been released. A remaining 24, including two of the ministers' sons, remain in custody, according to local media.
Details of the corruption allegations have not been made public, but are believed to relate to construction and real estate projects and Turkey's gold trade with Iran, according to Turkish newspaper reports citing prosecutors' documents.
The scandal has shaken investor confidence in Turkey before elections this year and heightened concern about the erosion of judicial independence, something which in the longer term could damage Ankara's bid for membership of the European Union.
According to SONAR research in the first major poll published since the scandal erupted, Erdogan's AK Party has seen its popularity slip to around 42 percent, down two percentage points from August; but it still remains comfortably ahead of the main opposition.
Local elections in March will be the first concrete test of the impact of the scandal, with the mayoral election in Istanbul, scene of weeks of anti-government demonstrations last summer, likely to be the most keenly-fought race.
Oz said he had been the victim of a smear campaign after allegations in pro-government media, which he denies, that he had taken a 77,500 lira ($35,500) holiday in Dubai last October paid for by a construction magnate targeted in the probe.
The High Council of Judges and Prosecutors (HSYK), the body responsible for appointments in the judiciary, has meanwhile launched an investigation into Oz's activities. ($1 = 2.1856 liras) (Additional reporting by Humeyra Pamuk; Editing by Nick Tattersall and Ralph Boulton)