Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginReq); 
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endReq);    
function beginReq(sender, args){     
// shows the Popup     
$find(ModalProgress).show();        
}
  
function endReq(sender, args) {    
 //  shows the Popup     
 $find(ModalProgress).hide(); 
 }