dojo.connect(form, "onsubmit", function(event){
    dojo.stopEvent(event);

    var digit_form = dijit.byId("user_profile_form");
    if (!digit_form.validate()) {
        return false;
    }
//  client-side validation is ok, so we submit form using AJAX

    var xhrArgs = {
        form: form,
        handleAs: "json",
        load: function(responseText){
            // here I get response from server
            // and if there are errors on server
            // responseText object contains array with errors, so I
            // need to show this errors to user 
        },
        error: function(error) {
        }
    }

    var deferred = dojo.xhrPost(xhrArgs);
}