* Officials at BDDK, TIB and editors in state media removed
* Experts say government aiming to block further leaks
* Government official signals removals to continue
* Several thousand people in three big cities protest (Updates with protests)
By Humeyra Pamuk
ANKARA, Jan 18 (Reuters) - Turkey has extended a purge of official bodies to the banking and telecoms regulators and state TV, firing dozens of executives in moves that appear to broaden Prime Minister Tayyip Erdogan's backlash against a corruption investigation.
The authorities have already reassigned thousands of police officers and about 20 prosecutors, and fired some state television officials in response to the corruption investigation, the biggest challenge to Erdogan's 11-year rule.
Investigators are believed to have been looking into allegations of corruption and bribery involving trade in gold with Iran and big real estate projects, although full details of their charges have not been made public.
The combative prime minister says the investigations, which began a month ago with arrests of high profile figures including the sons of three of his cabinet ministers, are part of an attempted "judicial coup".
His opponents say they fear a purge of official bodies will destroy the independence of the judiciary, police and media.
"It's like reformatting a computer. They are changing the whole system and people in various positions to protect the government," said Akin Unver, assistant professor of International Relations in Istanbul-based Kadir Has University.
Among dozens of officials dismissed in the latest sackings, Turkish media reported on Saturday that the deputy head of the banking watchdog BDDK and two department heads had been removed.
Five department chiefs were fired at the Telecommunications Directorate (TIB), a body that carries out electronic surveillance as well as serving as telecoms regulator, and a dozen people were fired at Turkey's state channel TRT, including department heads and senior news editors.
A government official said the firings were carried out for "the benefit of the public" and more could come: "Right now we are working on this issue and if we identify cases problematic to the public's benefit, more dismissals could be considered."
Pictures of money-counting machines and reports of cash stacked in the homes of people linked to the graft probe have caused uproar among the Turkish public.
Unver said the aim of the purge at the telecommunications watchdog could be to prevent further videos and pictures being published on the Internet.
"They are seeking a monolithic structure over the Internet," he said.
Several thousand people took to the streets in Turkey's biggest three cities on Saturday demonstrating against a government-led draft bill to increase controls over the Internet.
The bill would give the courts the power to rule on removing online material that "violates individual rights", an article that opponents say is murky and could lead to the arbitrary closure of websites.
In Istanbul's Taksim Square, where police fired teargas and water cannon to disperse the crowd, protesters called for the government to resign. Some chanted: "There are thieves around," referring to the corruption allegations.
SEIZURE OF ASSETS
Erdogan has suggested the graft inquiry, which has led to the resignation of three cabinet ministers and detention of businessmen close to the government, is an attempt to undermine his rule by Fethullah Gulen, a U.S.-based cleric with influence among the police and judiciary.
Earlier in the week the government reassigned twenty high-profile prosecutors, stepping up the purge of the judiciary.
Many of the people who have been fired are believed to be associated with the cleric's Hizmet movement, which claims more than a million followers and runs schools and charities throughout Turkey.
Gulen's lawyer says the cleric has nothing to do with the graft investigations, and his followers say they are victims of a witch hunt.
In a separate move denounced by the opposition as an attempt to target it, Turkish authorities have seized the assets of Mustafa Sarigul, the main opposition CHP party's mayoral candidate for Istanbul.
The state Savings Deposit Insurance Fund (TMSF) said it seized Sarigul's assets after he and his business partners failed to repay a loan dating back to 1998.
Sarigul denied he had an outstanding debt, describing the move as "a provocation".
"I have not received one single letter from TMSF in 16 years... Those who have lost the trust of the people and carried out this political attack on me will get the answer at the ballot box on March 30," he said, referring to local elections.
Erdogan, who has presided over an extended economic boom that has transformed Turkey and lifted millions of people from poverty, remains the country's most popular politician.
He and his moderate Islamist AK Party have long battled for influence against the secularist military establishment that dominated Turkey over the past century. Conflicts with the judiciary, police and Gulen followers add to his list of enemies.
It is still not clear what effect the crisis will have on Erdogan's political fortunes ahead of local elections approaching in March.
Last year saw mass street demonstrations among Turks who accuse the prime minister of authoritarianism, but those protests did little to undermine Erdogan's support among his conservative base of followers. (Additional reporting by Orhan Coskun and Mert Ozkan in Ankara, Ayla Jean Yackley in Istanbul, Reporting by Humeyra Pamuk; Editing by Peter Graff and Andrew Heavens)