Authorities are investigating Deputy Prime Minister Arinc on charges of misconduct and covering up felony, while Gokcek is being investigated for misconduct and embezzlement, Dogan news agency reported on Tuesday.
The two members of President Erdogan's AKP party had traded angry accusations on Twitter the day before, after apparent tensions between the government and the Turkish president.
Strike 'from within'
The rift started during the weekend, when Erdogan's longtime deputy Arinc asked the president to stop interfering and making "emotional" statements, after Erdogan criticized the government's handling of the Kurdish peace process.
"We love our president. We're aware of his power and the good service he gives to our nation. But there is a government in this country," Arinc said on Sunday.
But Recep Tayyip Erdogan, who had ruled Turkey as prime minister for 11 years before being elected head of state last year, said on Monday that it is "both his right and his duty" to voice his opinion.
Following this exchange, Ankara mayor Gokcek called on Arinc to resign. Gokcek, who is a staunch Erdogan supporter, accused Deputy Prime Minister Arinc of being an instrument of the "parallel state," referring to the so-called Gulen movement, which is allegedly plotting to overthrow the president.
"I must confess I wasn't expecting such a blow... they wanted to strike at us from within," Gokcek wrote.
In response, Arinc called the mayor "dishonorable" and threatened to reveal Gokcek's wrongdoings after the incoming election in June. He also admitted his sympathy for the Gulen movement but said he has been "standing by" his government and President Erdogan.
"I am not anybody's man. At the end of my political career I will walk with my head held high," he said.
Prosecutors would have to apply to Parliament to talk to Arinc, who has immunity as a Parliament member.
Constitution changes in store
Despite the friction at the top of his party, Erdogan still enjoys huge support among religious conservatives, and AKP appears on track to another victory.
Moreover, elections on June 7 could be crucial for Erdogan, who wants his party to take 400 of 550 seats in the Parliament. Thus, AKP lawmakers could change the constitution without a referendum and give the president more direct influence.
Erdogan has already chaired two Cabinet meetings after being elected president last August, and pressured the central bank over its monetary policy.
dj/jr (AFP, Reuters, dpa)