Turkish competition officials made surprise visits to several private, state and foreign banks late on Friday to analyse computers as they opened a wide-ranging probe of trading in the financial sector, according to three bankers.

Turkey's Competition Authority told Reuters on Tuesday it was conducting "preliminary research" into banks, as it regularly does in all sectors, adding it could not say more about the effort until the process is completed.

Turkish media reported on Friday that the competition authority, which is part of the Ministry of Trade, was investigating possible violations in foreign-exchange, deposit, credit and brokerage services.

Two Turkish broadcasters have said that more than 20 banks are involved.

Bankers said it was unclear exactly what the competition officials were investigating.

One banker said officials fanned out across Istanbul and simultaneously entered the lenders late on Friday afternoon. They sat down at trading desks to analyse computers, and requested more transaction information that is due this week, the person said.

"The investigation that started on Friday was a surprise to us," said a second banker, who also requested anonymity due to the sensitivity of the subject.

The competition authority was not working with the BDDK banking regulator, which usually does such audits and would have access to some of the information being sought, the second banker said.

"This is why it raises question marks since we don't exactly know what they are looking for," the person said. "All operations by banks are being investigated." The government has clamped down on financial markets with a series of new rules and regulations since a currency crisis in 2018 knocked nearly 30% off the value of the Turkish lira.

The changes - including curbs on foreign-exchange and reserve requirements meant to boost lending - were meant to stabilize the currency and kick-start a recovery from recession.

In 2013, the competition authority fined 12 banks a total of 1.1 billion lira ($620 million at that time) for collusion on interest rates.