Thank you for your response.
I already tried both:
The first option (On initial page load) - http://www.telerik.com/help/aspnet-ajax/ajax-show-loadingpanel-on-initial-pageload.html#seeAlsoToggle
The page finishes the loading process and I see the page's content, then the Loading gif shows up(after I saw the page content..)
and then it shows up again.
The second option (show/hide loading panel) - http://www.telerik.com/help/aspnet-ajax/ajax-show-hide-loadingpanel.html
The event "OnRequestStart" is not fired...
Do you have any ideas of how can I fix the first problem, that the page is showing up before the loading panel?