The head of Cologne's police force is leaving his post "to restore public trust in the police" following scores of attacks on women in the city on New Year's Eve.
The violence outside the main railway station has sparked a debate about Germany's open door policy on migrants.
Victims described chaos as the men carried out dozens of sexual assaults and robberies with little apparent response from the authorities.
Federal authorities say they have identified 18 asylum-seekers among 31 suspects linked to crimes committed in Cologne on New Year's Eve.
