Cologne city authorities said around 10,000 people on Saturday took part in marches organized by left-wing groups, AP reported.
Police were out in force amid concerns about possible violence but authorities said the event was peaceful.
In Paris, more than 1,000 gathered at the Place de la Republique to denounce Turkey's actions. Some displayed banners saying, "The Turkish state is committing crimes against humanity in total impunity."
Turkey's incursion into Syria, aiming to rid the border area of Syrian Kurdish fighters as US forces withdraw, has caused death and destruction and sent tens of thousands of civilians fleeing their homes.
In Cologne, some demonstrators carried placards with slogans such as "No deals with the AKP regime" — a reference to Turkish President Recep Tayyip Erdogan's party.
Germany is home to large Turkish and Kurdish communities, and tensions between them have turned violent in the past.