How to make Recaptcha a required field in MVC 3?

I have recaptcha on my signup form. The user must type something in the recaptcha box before it gets posted to the server. I have server side validation and client side validation for other fields.

Generated html from rcaptcha:

<input type="text" id="recaptcha_response_field" name="recaptcha_response_field" autocomplete="off" class="valid">

View:

@ReCaptcha.GetHtml(theme: "red", publicKey: GetPublicKey())
@Html.ValidationMessageFor(m => m.recaptcha_response_field)

Model:

[Required(ErrorMessage = "'captcha required' ")]
public string recaptcha_response_field { get; set; }

On load I see the 'captcha required' message. But if I type something it still appears. The form gets submitted with empty recaptcha box.

How can I make recaptcha required field at client side?

Thanks for helping

EDIT: I added this to intercept the submit btn click event, but it doesn't stop from posting the form.

    <button type="submit" id="btnSubmit" class="sprt bg_red bt_red h25" onsubmit="ValidateAndSubmit();">Signup</button>
<script type="text/javascript">    
    function ValidateAndSubmit() {
            var v=$('#recaptcha_response_field').val();
            if (v == '' || v == undefined) {
                alert('captcha is required');
                return false;
            }
            return tr开发者_如何转开发ue;
        }
</script>

to make it work as you type it, return the value, not just call it. Type

onsubmit="return ValidateAndSubmit();"

or

onclick="return ValidateAndSubmit();"

Also you can try to add the same validation on the form tag.

Though this question is pretty old, thought it it might help to share my solution, that supports MVC unobtrusive javascript.

if ($("#recaptcha_response_field").length > 0) {
  $("#recaptcha_response_field").attr("data-val", "true")
    .attr("data-val-required", "The captcha field is required.");
  $("<span />")
    .attr("data-valmsg-replace", "true")
    .attr("data-valmsg-for", "recaptcha_response_field")
    .addClass("field-validation-valid")
    .appendTo($("#recaptcha_response_field").parents(".editor-field"));
}