try this:
<asp:updatepanel id="upd" runat="server">
<contenttemplate>
<asp:textbox id="TextBox1" runat="server"></asp:textbox>
<asp:button id="Button1" runat="server" text="Button" />
<asp:requiredfieldvalidator id="RequiredFieldValidator1"
runat="server"
ControlToValidate="TextBox1"
ErrorMessage="RequiredFieldValidator">
</asp:requiredfieldvalidator>
</contenttemplate>
</asp:updatepanel>
Good luck