Assign New Class To A Div To Show Errors If Any?

im trying to get the errors of an upload form validation displayed in a div.

<div id="error_div" class=""></div>

Here is the javascript:

<script type="text/javascript" language="javascript">
function validateForm(){
var flag = true;[code]....

Instead of showing the errors with an alert, i want to show them on the div and at the same time changing its class atribute to Alert, wich is a class i defined on my css page.Mybe with this?

document.getElementById(error_div).className='Alert';