procedure TForm1.btnWriteClick(Sender: TObject);
var
  jo, joi: TJSONObject;
  ja: TJSONArray;
  //jv: TJSONValue;
  id, nam, jsonString: string;
  I: Integer;
begin
  jo := TJSONObject.Create;
  jo.AddPair('fld_1', 'поле1');
  jo.AddPair('fld_2', 'поле2');

  ja := TJSONArray.Create;

  for I := 1 to 3 do
    begin
      joi := TJSONObject.Create;
      joi.AddPair('id',i.ToString);
      joi.AddPair('nam','nam' + i.ToString);
      ja.AddElement(joi);
    end;

  jo.AddPair('items', ja);

  ShowMessage(jo.ToString);
end;