Web method call fails when authorization is required - 'namespace' is undefined OR 'Authentication failed'

Hello everybody, I try to call a web-service method before authorization (when latter is required) on my login page.

But when I call it through a proxy, like this -
 MyNamespace.WebService.Login("name", "passw", false, OnComplete, OnTimeOut, OnError
); I get runtime error on this line - 'MyNamespace' is undefined. And when I call it using WebServiceProxy.invoke(), like this -   Sys.Net.WebServiceProxy.invoke("/WebService.asmx", "Login", false,
{"userName":strUserName, "password":strPassword, "createPersistentCookie":false}, ...