A few hundred protesters marched through central Athens on Tuesday, as NATO Secretary General Jens Stoltenberg met with Greek officials.
The protesters held up banners bearing messages such as "NATO killers are going home."
They marched in front of the parliament building before peacefully ending the rally.
Stoltenberg was in Athens on a one-day visit and met with Greek Prime Minister Kyriakos Mitsotakis.
The two discussed recent tensions in the eastern Mediterranean where Greece and Turkey are in a dispute over maritime borders.
Stoltenberg visited Athens a day after meeting in Ankara with Turkish government officials, including President Recep Tayyip Erdogan and Foreign Minister Mevlut Cavusoglu.