Flavio B., 71, is accused of sexually abusing boys during training over a period of 10 years, the Swiss news agency SDA reported.
The 20 Minuten website said the case involved around 15 boys aged under 16, according to court documents.
Most of the around 400 incidents occurred in Bellinzona, the website said.
Addressing the court on the first day of his trial the former president of the Bellinzona swimming club denied that he had ever put threatened or used violence against his underage victims.
“They were my friends. We always had a very close, trusting relationship,” the coach said.
He said the friendships had then taken on a sexual dimension.
Asked by the judge if he had never, even in retrospect, considered the possibility that the boys from the swimming club had only agreed to his advances because of his position of power, the accused replied “no”.
Several victims testified, however, that they had been afraid of the instructor and had not dared to tell anyone about the occurrences.
The victims said in written testimony that they would have avoided sexual contact with the instructor if they had been able to.
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B., who was Swiss national coach in the 1980s, was arrested in December 2011 after two former victims brought charges against him.
The prosecutor’s office made the case public to encourage other possible victims to come forward, 20 Minuten said.
B. was suspended from his positions with European and international swimming organisations after his arrest, the website said.