<asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1"
 DisplayAfter="1">
<ProgressTemplate>
<div id="update">
</div>
<div id="processMessage">
Elaborazione in corso... <br />
</div>
</ProgressTemplate>
</asp:UpdateProgress>


il css

#update
{
  position:absolute;
    top:0px;
    bottom:0px;
    left:0px;
    right:0px;
    overflow:hidden;
    padding:0;
    margin:0;
    background-color:#ffffff; 
    filter:alpha(opacity=50);
    opacity:0.5;
    z-index:1000; 
}

#processMessage { 
  position:absolute; 
  top:30%; 
  left:43%;
  padding:10px;
  width:20%;
  z-index:1001;
  background-color:#ffffff;
  border: solid 2px #336699;
  color: #336699;
  font-size: larger;
  font-weight: bold;
  text-align: center;
}