jquery popup not working

<script src="Scripts/jquery-ui.js" type="text/javascript"></script>
   <script src="Scripts/jquery-1.8.2.js" type="text/javascript"></script>
   <script type="text/javascript">
       function showmodalpopup() {
           $("#popupdiv").dialog({
               title: "jQuery Popup from Server Side",
               width: 430,
               height: 250,
               modal: true,
               buttons: {
                   Close: function () {
                       $(this).dialog('close');
                   }
               }
           });
       };
   </script>

<form id="form1" runat="server">
   <div>
       <asp:TextBox runat="server" ID="txtText"></asp:TextBox>
       <asp:UpdatePanel>
           <ContentTemplate>
               <div id="popupdiv" title="Basic modal dialog" style="display: none">
                   <asp:Label runat="server" ID="lblTextboxValue"></asp:Label>
               </div>
           </ContentTemplate>
       </asp:UpdatePanel>
       <asp:Button ID="btnShowModal" runat="server" Text="Show Popup" OnClick="btnShowModal_Click" />
   </div>
   </form>

protected void btnShowModal_Click(object sender, EventArgs e)
  {
      lblTextboxValue.Text = txtText.Text;
      //ScriptManager.RegisterStartupScript(this, GetType(), "Show Modal Popup", "showmodalpopup();", true);
      ClientScript.RegisterStartupScript(this.GetType(), "Popup", "showmodalpopup();", true);

  }