Hundreds of people from around the world are taking part in the World Winter Swimming Championship held in the UK for the first time.
Competitors have travelled from around the world
More than 600 swimmers representing 21 countries are competing in the competition, held this year at the Tooting Bec Lido in south London.
The official championship race is the 25-metre "head-up breaststroke".
The fifth biannual event is the first to be held outside Finland, where the championship was launched in 2000.
Margy Sullivan, vice president of the South London Swimming Club, said: "The Finns decided to make the races 'head-up breaststroke' to give the amateurs a chance against the elite.
"The championship is less about speed and more about coping with the cold, the challenge of the races and having some fun."
The two-day event also involves non-competitive swims, a 60-metre freestyle fun race, an endurance swim, international relay races and a water performance competition.
Participants have travelled from countries including Russia, Slovakia, United States, Latvia, Italy, Ireland, Finland, Estonia, Germany, Australia and Canada.
A special sauna and two hot "geyser wooden bath barrels" were imported for the event.
The 101-year-old lido in Tooting Bec is the largest fresh water swimming pool in Europe and winner of the Golden Goggles Award 2007.