Forty Austrian men are also involved, judicial authorities in the northeastern Swiss canton of St. Gallen said on Wednesday. Four Swiss men have been arrested for "acts of a sexual nature" against children, and for producing pornographic films and images involving young girls.
Legal proceedings have also been launched against another nine Swiss men aged between 20 and 65. Two of the men are accused of sexually abusing a 13-year-old girl and posting images of the abuse on the internet forum.
The homes of the 13 men have been searched by police, and computer hard-drives and other equipment have been confiscated. Austrian police said Wednesday they had opened a case against 15 men between 30 and 60 years old following the break-up of the paedophile network.
Police spokesman Helmut Greiner said Swiss authorities had sent the IP addresses of 35 Austrians to their colleagues in Vienna in connection with the probe in Switzerland.
Cyber-crime investigators uncovered the network which was hosted by a St. Gallen-based Internet service provider. Investigating magistrate Ursula Brasey told Swiss news agency ATS that the head of the ISP had alerted the authorities to the content of the site, and would not face any charges.
The German-language forum featured accounts from different men about their experiences with young girls and how best to 'groom' them for pornographic ends, the authorities said.