Error Page

I have created a custom error page and if the incorrect error url is of the form mysite.com/?q=node/xxx the page works fine. My site, however replaces an old site at this url so I need to be able to redirect urls of the form

mysite.com/events/conferences.lasso

OR

mysite.com/about/publications/listing.lasso

currently those urls just error to the standard Object Not Found 404 page.

Any thoughts about how to redirect any non-existing page at my url to the custom error page?

thanks