Dozens of attendees have reported incidences of the virus, and the Finnish Communications Regulatory Authority has issued an alert.
Local telecoms operator TeliaSonera has set up a special centre at the stadium to deal with infections, and is giving away time-limited antivirus software to those affected.
"In order to prevent mobile malware from spreading at the event, we have provided some powerful security software," said Pasi Mehtonen, vice president of mobile services at TeliaSonera.
"We have already taken measures earlier this year to prevent any known viruses from infecting TeliaSonera customers through MMS messages."
The Cabir virus spreads by Bluetooth, finding other Symbian Series 60 phones in the area and sending a file that contains the virus. The user has to accept and install the file in order for the code to activate.
While Cabir contains no malicious payload, the virus will wear down batteries very quickly since it constantly tries to broadcast itself onwards. Switching off Bluetooth blocks transmission of the virus.
"This happens easily when you gather tens of thousands of people from all over the world to a very small area," said Mikko Hyppönen, director of antivirus research at F-Secure, which is providing the antivirus software.
"To some extent the same thing happened during the Live 8 concerts earlier this summer. We now have staff at the Olympic Stadium in Helsinki assisting visitors in cleaning out affected phones."
Cabir was the first example of a mobile virus and was created as a proof-of-concept by the infamous Russian hacking group known as 29a.
It was created in June 2004, first detected in the wild by August of that year and now appears regularly around the world, carried by air travellers.
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