Thousands of amateur swimmers have taken to the brackish waters of Rio de Janeiro's Copacabana beach for an open-sea race.

Activists from a group calling for large-scale sanitation projects also turned out for Saturday's King and Queen of the Sea competition, which brings together thousands of amateurs and a small group of elite swimmers.

The two-day-long contest took place near the south end of Copacabana where the marathon swimming event will be held during the 2016 Olympics in Rio.

Water pollution has become a hot-button issue here with 70 percent of sewage in this city of 6 million flowing untreated into waterways and just 2 ½ years to go before the start of the games.

Some participants at Saturday's event complained about the water's dirtiness.