I want to apply server side validation using custom control validator.
Do something like this
<asp:CustomValidator ID="valCaptcha" runat="server" ControlToValidate="txt1"
ErrorMessage="Your error message here" Text="*" OnServerValidate="valCaptcha_ServerValidate" ValidateEmptyText="True" />
protected void valCaptcha_ServerValidate(object source, ServerValidateEventArgs args) {
}
Correct me if I understand you wrong.