How to run long running operation asynchronously and show loading for the user in ASP.Net Ajax and poll for the result?

Question

I've a heavy operation which takes long time to be completed in my asp.net application. I don't want to run the entire operation in one request which may result in a TimeOut error. Instead I want to invoke the operation in a separate thread and poll for the result every x seconds. How can I do this?

If the operation gets completed I need to register a script in the ajax postback to hide the loading panel and show the content. However I'm not able to register new scripts and invoke it during ajax postbacks.

Any ideas?