function beginReq(sender, args){ 
    $find(ModalProgress).show();        
} 
 
function endReq(sender, args) { 
    $find(ModalProgress).hide(); 
}

Sys.WebForms.PageRequestManager.getInstance().add_beginRequest(beginReq);
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(endReq);