Sys.WebForms.PageRequestManager.getInstance().add_endRequest(RequestCompleted);

    Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(RequestStarted);


        function RequestStarted(sender, args){
            $("#divOverlay").show();
            $(".divLoading").css("top",($(window).height() / 2) + $(window).scrollTop());
        }

        function RequestCompleted(sender, args) {
            $("#divOverlay").hide();
        }