Julien Brauers, 25, told Bild newspaper that a guard with security firm ISS, who was stationed at a lift near the main entrance, went so far as to remark that "disabled persons pose a safety risk."
"I had always been allowed to use the handicap-accessible toilets there," said Brauers, who has been severely disabled since birth.
The town hall was swamped with people attending an official reception on Sunday, and Brauers's friend wheeled him from door to door but was repeatedly barred from entering the building.
Stephan Pieper, general manager of ISS security, defended his staff. "Our employees were instructed not to make any exceptions - however, they did point out the public toilets nearby," he told Bild.
Ultimately, Brauers had to be taken to a much smaller normal toilet by two friends.
Dino Conti-Mica, security liaison for the city's Karneval committee, said the organisation was looking into the situation and trying to contact those responsible. "I am very embarrassed by the incident," he said.
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Düsseldorf Mayor Dirk Elbers told Bild that he was not aware of the incident. "I can stress, however, that what happened is by no means in line with my intentions," he said.