ISTANBUL (AFP) - Anti-terror officers in Istanbul detained 29 suspected members of the Islamic State in Iraq and Syria (ISIS) group who were allegedly preparing a "sensational attack" in Turkey, police said Friday (July 7).

Twenty-two foreigners were among those detained in overnight raids, the Istanbul police headquarters said, without giving details of their nationalities.

Police seized several electronic devices and one gun during the operation.

The 29 detained, many of whom had fought in war zones, were part of an ISIS cell about to commit a "sensational attack" in Turkey, police said without giving further details.

Turkish police on Wednesday detained six suspected members of ISIS planning a bomb attack on an opposition protest march led by the Republican People's Party (CHP) leader Kemal Kilicdaroglu.

He is on the final stage of a 450-kilometre (280-mile) "justice march" from Ankara to Istanbul to protest over the legal system under President Recep Tayyip Erdogan.

The Turkish border had previously been a key transit point for ISIS recruits and supplies before Turkey tightened security and launched a military operation to clear the jihadists from its southern border last August.

Turkey suffered a series of terror attacks in 2016 and one in 2017 blamed on ISIS.

The last attack - claimed by ISIS - was during New Year's celebrations at the elite Istanbul nightclub Reina when a gunman killed 39 people.