A woman reporting from the city center for a Belgian broadcaster was groped live on television, and police are searching for a suspect.
The city has been on high alert for sexual assault after hundreds of women filed criminal complaints of having been groped on New Year's Eve
by men described as being of North African or Arabic origin. A handful of the complaints alleged rape.
But Thursday's incident during a celebration called Women's Carnival does not appear to be related. The men taunting Esmeralda Labye in the lead-up to the harassment appear to be of European origin.
"I'd like to emphasize, and this seems important me, that the incident has been caused by young men speaking German," she said.
As Labye delivered her live report for RTBF, men behind her began making obscene gestures. A man kissed her on the neck, she said. Then someone whispered a proposition into her ear.
Because of the heightened police presence for the massive public event, in which tens of thousands of revelers celebrate and alcohol consumption is high, she was not concerned things would get out of hand.
"As I was waiting to wrap up my live, one of the three men touched my breast. At that moment, I lost my temper. I turned and told them in English: 'I won't allow this to happen twice! Don't touch me. Don't touch me.'"
Three men standing around her left, she said.
RTBF, a CNN affiliate,
has decided not to re-broadcast the video of the incident but has published two still images. The station filed a criminal complaint with Cologne police to help expedite the investigation.
"The station expresses respect and support for its colleague journalist and disgust and condemnation for the attackers," RTBF said in a statement.
After the incident, Labye decided to finish out her work day, RTBF said. "Cologne's mayor's office apologized to the station for these acts and gave its support to our colleague journalist and promised police security if needed."
A 17-year-old boy recognized himself in the shot behind Labye and turned himself in to police in the company of his mother, Cologne police said, but he said he had not harassed the reporter. After reviewing the footage, police decided it corroborated his claim and let him go.
Carnival is celebrated in many traditionally Catholic regions.
It is intended as a last goodbye from the pleasures of the flesh before the onset of Lent, a time in which observant Catholics give up some pleasures, or fast, to reflect the biblical story of Jesus' fasting in the desert before beginning his public ministry.
But the celebration has grown in many places into a bawdy secular street party.
During Thursday's Women's Carnival, police in Cologne, a city of roughly 1 million residents, lowered its threshold for stepping in during possible incidents and detained at least 190 people.
"With the rise in alcohol consumption, the potential for aggression unfortunately also rose in the evening," police said. "Police forces on duty had to step in more and more to prevent escalation."
Police registered more than 220 criminal complaints, most of which involved personal injury. At least 18 complaints were sexual in nature, ranging from sexual insult to a possible attempted rape.