Show Progress bar

 <script language="javascript" type="text/javascript">
        function success_handler(o) {
            replace_html('content', o.responseText);
        }

        function replace_html(id, content) {
            document.getElementById(id).innerHTML = content;
        }

        function show_progressbar(id) {
            //replace_html(id, '<img src="" border="0" alt="Loading, please wait..." />');
            replace_html(id, '<asp:label id="aa" runat="server" text="Loading,please wait....." style="border:none;font-size:12px;font-weight:bold;" />');
        }

        function send_request() {
            show_progressbar('content');
            var callback = { success: success_handler };          
        }  

</script>

  <div id="content" align="center">
                        </div>

// this button will call the "send_request( )"


 <asp:Button ID="btn_submit" OnClientClick="javascript:send_request();"  Text="Search" runat="server" Width="120px" />