ATHENS (Reuters) - Greek anti-terrorism police found artillery and a secret tunnel during a raid on two premises in the Greek capital Athens on Thursday, authorities said.

More than 20 people, non Greeks, were detained by anti-terrorist police. The raids, still underway, were taking place in the Athens neighborhood of Sepolia and in the Exarchia area of central Athens.

The operation was launched after a tip-off, a police source said, adding that authorities were investigating possible links with a militant organization Turkey has outlawed.

In November 2017, eight men and a woman were arrested by Greek police, days before an expected state visit by Turkish President Tayyip Erdogan. A Greek court acquitted them of any terrorism-related offences in May 2019, due to lack of evidence.