“We can only urgently advise people against dressing in such costumes at the big events,” a spokesperson for Monchengladbach police told the Rheinische Post.
“If someone wants to dress like that at his own party, please go ahead - but please don't do it in public,” the speaker appealed.
Düsseldorf police also said they would be pleased “if people would do without such clothing.”
At carnival street parties, which are held across much of western and southern Germany, it is customary to dress up.
Fears have been raised this year though about the safety of the event after a New Year's street party was cancelled in Munich, when police were given a tip-off about a possible bomb threat.
Police are not just worried about how people dress but also them taking fake weapons to accompany their costumes, and have warned people against bringing weapons if they dress up as cowboys or Indians.
“The sometimes look completely real. We're trained to know the difference but you still have to get up really close before you can judge it,” one policeman told the RP.
The festival organizers told the RP that they felt no need to tell party-goers how to dress.
“Our revellers don't need to be told. We assume they're sensible enough to know already,” Sigrid Krebs from the Cologne Carnival Committee said.