The event, organised by the group Fluss Bad Berlin, was registered as a protest to bypass a long-standing ban on swimming in the river, in place since 1925 due to past pollution.
Campaigners say water quality has improved significantly and is regularly monitored, with city officials considering reopening parts of the river to swimmers by 2026.
The group proposes a 1.8-kilometre boat-free stretch near Museum Island, citing Paris’ reopening of the Seine as a model.