How to prevent show loder image and prevent bootstrap modal dialog box from closing while AJAX response is coming from PHP file?

I'm using Bootstrap v 3.0.0 I've following HTML code of Bootstrap Modal:

<div class="modal fade" id="newModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true">
      <div class="modal-dialog">
        <div class="modal-content">
          <div class="modal-header">
            <button type="button" class="close" data-dismiss="modal" aria-hidden="true">&times;</button>
            <h4 class="modal-title">Submit Form</h4>
          </div>
          <div class="modal-body" style="max-height: 300px; overflow-y: auto;">
            <br/>
            <!-- The form is placed inside the body of modal -->
            <form id="request_form" method="post" class="form-horizontal" enctype="multipart/form-data" action="">
              <div class="form-group">
                <label class="col-sm-4 col-sm-offset-1 control-label">Reg ID <span style="color:#FF0000">*</span> :</label>
                <div class="col-sm-5">
                  <input type="text" class="form-control" name="reg_id" id="reg_id"/>
                </div>
              </div>
              <div class="form-group">
                <label class="col-sm-4 col-sm-offset-1 control-label">Reg Date<span style="color:#FF0000">*</span> :</label>
                <div class="col-sm-5">
                  <input type="text" class="form-control date_control" id="reg_date" name="reg_date" value="" placeholder="yyyy-mm-dd">
                </div>
              </div>
              <div class="form-group">
                <label class="col-sm-4 col-sm-offset-1 control-label">Upload Image<span style="color:#FF0000">*</span> :</label>
                <div class="col-sm-5">                   
                  <input type="file" name="reg_image" id="reg_image" accept="image/*" capture="camera" />                  
                </div>
              </div>  
              <div class="form-group">
                <div class="col-sm-5 col-sm-offset-5">
                  <button type="submit" class="btn btn-success" id="btn_receipt_submit">Submit</button>
                  <button type="button" class="btn btn-danger" data-dismiss="modal">Cancel</button>
                </div>
              </div>
            </form>
          </div>
        </div>
      </div>
    </div>

For above bootstrap modal I've written following AJAX-jQuery code :

$('#request_form').submit(function(e) {
  var form = $(this);
  var formdata = false;
  var reg_id = $('#reg_id').val();
  var reg_date = $('#reg_date').val();

  if(window.FormData) {
    formdata = new FormData(form[0]);
  }

  var formAction = form.attr('action');

  $.ajax({
    url         : 'xyz.php',
    type        : 'POST',    
    cache       : false,
    data        : formdata ? formdata : form.serialize(),
    contentType : false,
    processData : false,
    beforeSend: function() { 
      $(".btn").prop('disabled', true); // disable both the buttons on modal      
    },

    success: function(response) {
      $(".btn").prop('disabled', false); // enable both the buttons on modal
      var responseObject = $.parseJSON(response);    
      if(responseObject.error_message) {
        if ($(".alert-dismissible")[0]) {
          $('.alert-dismissible').remove();   
        }  
        var htmlString = "<div class='alert alert-danger alert-dismissible' role='alert'><button type='button' class='close' data-dismiss='alert' aria-hidden='true'>&times;</button>"+responseObject.error_message+"</div>";    
        $(htmlString).insertBefore('div.modal-body #request_form');        
      } else {
        $('#newModal').modal('hide');
        $('#Modal2').modal('show');       
      }
    }
  });
  e.preventDefault();
});

I want to show some loader image at the the center of the screen when the AJAX request goes to PHP file and it should be displayed at the center of the screen until the response from PHP file comes.

Also during this time user should not be able to close the modal, nor it should get hide if user clicks anywhere apart from the modal.

In short user should not be able to click anywhere, nothing should happen until response for AJAX request comes from PHP file.

I tried many tricks but I'm able to disable the two buttons appearing on form until the response comes. But actually I want to do much more than this.

Can someone please help me in this regard please?