With a starter in his hand, Prince Harry said "take your mark" and pressed a button, and a group of local swimmers were off.
The British prince was visiting a private tennis club in Belo Horizonte where Britain's Olympic team will be based ahead of the 2016 Rio de Janeiro Games. Later, he will attend England's final World Cup group match against Costa Rica on Tuesday at Mineirao Stadium.
A few hundred schoolchildren were bused in for the event Tuesday and cheered the prince's arrival, which was watched by residents looking out their window from adjacent apartment buildings. After the race, Harry placed medals over the heads of the swimmers, some of them children.
The Minas Tenis Clube has an Olympic-size pool, tennis courts and an arena where judo and other sports can be practiced.