Disable button and show loading message while user clicked in asp.net c# USING UpdatePanel

After the ScriptManager put follow script on the page
 
<script type="text/javascript">
var pbControl = null;
var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_beginRequest(BeginRequestHandler);
prm.add_endRequest(EndRequestHandler);
function BeginRequestHandler(sender, args) {
pbControl = args.get_postBackElement(); //the control causing the postback
pbControl.disabled = true;
}
function EndRequestHandler(sender, args) {
pbControl.disabled = false;
pbControl = null;
}
</script>

Ref1, Ref2