The demonstrators gathered in response to a call on Facebook to gather in front of the Cologne Cathedral and produce as many soap bubbles as possible. And the in crisp autumn weather, hundreds of residents of the Rhine metropole flocked to the city centre to take part, Die Welt reports.

The protest was sparked by a perceived ban by the city's public order office on street artists blowing bubbles formed out of soap suds.

In recent weeks, street artists have been enchanting crowds in the city centre by blowing enormous soap bubbles at rapid speed through the air, the Kölner Stadt-Anzeiger reports.

Authorities claim the bubbles “contaminate public spaces” since they contain detergent that could end up in the city’s drinking water reservoirs.

The soap also places a film on the streets which leads to an “acute danger” of pedestrians slipping and injuring themselves, leading public order officials to threaten street artists with fines of up to €510, the paper reports. 

Around 2,000 people signed up for the protest on Facebook, with one commentator describing the regulation as “unbelievable pettiness”.

But the city's public order office refuted the idea that it had laid down a wholesale ban on soap bubbles.

Shop owners had complained that the soap suds were dirtying their windows and making the entrances to their shops slippery, public order office boss Engelbert Rummel told Express.

“We asked the street performers to go somewhere else. But there was no ban imposed,” he insisted.

Cologne mayor Henriette Reker has also waded into the slippery debate, insisting that a ban would be ridiculous.

“That is absolute nonsense,” she told Express. “This beautiful and fantastical enrichment of our city is not banned. Where would we end up otherwise?”