In scenes reminiscent of this summer's massive anti-government revolts, hundreds of people took to the streets in cities across Turkey on Friday night calling for the government to resign following a high-profile corruption scandal that involves sons of cabinet ministers, leading businessmen and the head of a state-owned bank.

In Istanbul, riot police broke up demonstrations using teargas, water cannon and plastic bullets. According to Turkish media reports, 70 people have been arrested. Protesters chanted "catch the thief", in reference to a highly political corruption probe that started with orchestrated dawn raids on 17 December and is continuing to send shock waves through Turkey, edging ever closer to the heart of the Turkish government.

Seen by many as the most serious challenge to the 11-year rule of Turkish prime minister Recep Tayyip Erdogan, the corruption investigation has targeted persons known to be close to the government of the Justice and Development party (AKP).

Three ministers were forced to resign when police detained their sons following a long-running investigation into allegations of corruption. Two of the sons are still in custody along with 22 others awaiting trial, facing accusations of corrupt practices, including bribery, tender rigging and illicit money transfers to Iran.

Erdogan remained defiant in the face of the crisis, repeatedly blaming a "conspiracy" for the corruption investigation that he called a "dirty operation". He reshuffled his cabinet on Wednesday, replacing 10 ministers with names considered to be loyal to the line of the AKP.

Many domestic commentators believe that the probe is a sign of increasing discord within the country's conservative power base, between the AKP government and its former moderate Islamist allies, the so-called Hizmet movement, led by influential exiled cleric Fethullah Gülen, who is based in the US.

While clouds of teargas rose once again from the centre of Istanbul on Friday night, the prime minister held a defiant rally at the city's airport, where he stressed that he would refuse to step down over this crisis. He repeated his earlier allegations that the inquiry was unjustified. "Those who called this operation a corruption operation are themselves the very ones who are corrupt," Erdogan said.

He also harshly rebuked three MPs, including former tourism minister Ertugrul Güney, who had resigned from the AKP on Friday in protest over growing accusations against the government for interfering with the investigation.

Scores of senior police officers and judiciary officials have been removed from their posts after the arrests, a move slammed by opposition parties and critics as an attempt to cover up the burgeoning political scandal.

"I have never come across such blatant government meddling with the judiciary before", said Sezgin Tanrikulu, deputy head of the main opposition People's Republican party (CHP), a lawyer and former head of the Diyarbakir Bar Association. "This is highly worrying. The little trust that people had left in the Turkish justice system is now gone."

On Thursday, public prosecutor Muammer Akkas, who was overseeing part of the inquiry, was also taken off the case, allegedly for leaking information to the press and for failing to keep his superiors sufficiently informed. However, in a letter to the Turkish media he accused the government of actively hindering the investigation. "The judiciary was subjected to open pressure by the police force, and the execution of court orders was obstructed," he said.

Last week police officers refused to comply with the prosecutor's orders to take more suspects into custody, a list of 41 high-ranking businessmen and officials, according to the Turkish daily Hürriyet.

"This is very disturbing," Sezgin Tanrikulu said. "These officers committed a serious crime by refusing to follow their orders. There are legal channels to contest a prosecutor's decisions, but it is not the place of the police to do so."

He added: "This dealt a serious blow to the investigation. It gave the suspects time and opportunity to tamper with the evidence, even to flee Turkey. It is the first time that I know of that the police illegally interfered in an ongoing investigation."

Tanrikulu underlined that the officers' refusal had come after the removal of hundreds of policemen from their posts, including the Istanbul police chief Hüseyin Çapkin, and new appointments to the Turkish police force.

Meanwhile, a proposed judicial regulation that would have required police investigators to keep their superiors informed at all times was blocked by Turkey's highest administrative court as unconstitutional.

"It would have meant that investigators would have had to inform former interior minister Muammer Güler of the ongoing corruption probe against his son," lawyer Burcu Öztoprak said. "It would have been the end of the separation of powers." Lauding the administrative court's decision, she added that the proposal of the new regulation alone was a worrying sign for the state of Turkey's justice system.

"It shows under how much pressure Turkish courts currently are. Judges and prosecutors cannot make independent decisions any longer", she said. "The government sends out the signal that nothing in this country can be done without their knowledge any more, that absolutely everything needs to be under their control."