Inspectors raided the head offices of National Bank, Piraeus Bank, Eurobank, Alpha Bank, Attica Bank and the offices of the Greek bank association, bankers said on Thursday.

They said the competition authority, which is an independent body, had carried out the unprecedented raids because it was looking into whether banks had colluded on transaction fees.

"The checks reflected the Competition Commission's concerns that the businesses under investigation may have been involved in anti-competitive practices of horizontal collaboration," the authority said in a statement on Friday.

It said the raids, which it had been planning for weeks, were preliminary steps in cases where there may be indications of anti-competitive practices.

"Conducting such probes does not prejudge that firms being investigated have indeed been involved in anti-competitive behaviour, nor the probe's outcome," the authority said.

Some bankers told Reuters on Thursday that the raids were politically motivated.

Last month, Prime Minister Kyriakos Mitsotakis urged banks to revoke increases in charges for a series of services.

On Friday, Development Minister Adonis Georgiadis said the government had nothing to do with the raids, saying the authority was an independent body.

"I assure you that the ministry did not have the slightest involvement in yesterday's developments," he told Skai TV. "I was not aware, I should not have been aware and, fortunately, I did not know about it."

(Reporting by George Georgiopoulos and Renee Maltezou; Editing by David Clarke)