Turkish PM Recep Tayyip Erdogan says he will not step down amid corruption allegations that have forced the resignations of three ministers.
The ministers' sons were among dozens of people detained as part of a wide-ranging corruption probe.
Turkey's army has said it does not want to get involved in political arguments, in response to rumours of a coup plot.
On Friday police fired tear gas and rubber bullets to break up protests in Istanbul's Taksim Square.
Protests were also held in the capital Ankara and the third-largest city Izmir.
Protesters had chanted "catch the thief" and called for Mr Erdogan's resignation.
Police are investigating allegations of illicit money transfers to Iran and bribery for construction projects.
The scandal has caused Turkey's lira to fall to a new low against the dollar.
At a rally outside at Istanbul's main airport late on Friday, Mr Erdogan pledged to fight on in what is seen as the biggest challenge to his government in his 11 years in office.
He called the investigation into corruption a "smear campaign" and urged thousands of supporters to vote for his party in local elections next month.
"Those who call it a corruption inquiry are corrupt themselves," he said.
Turkey has a history of military takeovers but its power has been curbed during Mr Erdogan's decade in power.
The country's top administrative court has blocked a government decree, introduced last week, ordering police to inform their superiors before launching investigations.
The Council of State's ruling on Friday, blocking the regulation, is seen as the latest setback in Mr Erdogan's efforts to stop the political fallout from the scandal: Three politicians resigned from the governing Justice and Development Party (AKP) over the row.
Numerous police and judicial officials have been removed from their posts since the first arrests 10 days ago.
On Wednesday Mr Erdogan named 10 new ministers to join his cabinet after talks with President Abdullah Gul.
Commentators in Turkey believe the scandal stems from a power struggle between Mr Erdogan's government and an influential US-based Muslim cleric, Fethullah Gulen, who is said to have many followers within Turkey's police and judiciary.
Supporters regard the Hizmet movement inspired by Mr Gulen as the benign, modern face of Islam, but critics question its motives.