Block all actions on a page

Block all actions on a page

hi,
i think this is a easy task, but i can't find a solution:
i built a application with the jquery-plugin jstree to maniputate a tree stored
in a database. all actions like create, move, delete and rename are fire up
ajax-calls to php-scripts with mysql-querys. i got it to work, to display a
trobber and 'waiting'-text during the ajax-calls, but what i also want to do is
"blocking" the site for this time, so that the user HAS TO wait and couldn't
click somewhere else. This blocking should be should be visible like the
behaviour of modalwindows. the background should be with a dark overlay.
sorry for my bad english,
thank you for tipps and
best,
heinetz