Organizers said the demonstration on Saturday night was aimed in part at protesting against Turkey's military intervention in northern Syria and what they call the "dictatorial" behavior of Turkish President Recep Tayyip Erdogan.
They argued that Ocalan, who has been in prison in Turkey since 1999, should be recognized as a "legitimate negotiating partner."
Cologne police put more than 1,000 officers in place. City police chief Juergen Mathies said that pictures of Ocalan were allowed but demonstrators would not be allowed to show symbols of the banned PKK, or Kurdistan Workers' Party, the Associated Press reported.
Turnout at Saturday's event was estimated at close to 30,000, and there were no reports of significant trouble.
A month ago, up to 40,000 people rallied at the same site to denounce the attempted military coup in Turkey and show support for Erdogan.
The PKK, which wants greater autonomy for Kurds living in Turkey, is considered a terrorist group by Turkey, the US and the European Union.
A ceasefire between Turkey and the PKK collapsed last year. Last month, Turkey launched a cross-border operation whose aims include stalling Syrian Kurdish militants from seizing more ground in northern Syria.