STORY: "Copenhagen harbour has one of the cleanest harbour waters in northern Europe, and I think it's really important that we use it for leisure," Mayor for Culture and Leisure in Copenhagen, Mia Nyegaard said.

The night swim was not competitive. Lifeguards on bodyboards and in boats were on standby.

The event formed part of the annual TrygFonden Copenhagen Swim, a series of competitive and recreational swimming events that take place over two days in Copenhagen's harbour.

"My brother - he used to love open-water swimming and he passed away and he told me before he passed away, he wanted me to keep going, keep swimming and keep doing it so he'd have been here in a heartbeat if he could have been, so I wanted to do it in his memory," said participant Becky Morris.