What you are probably doing is setting the default action to shown when the default action should be hidden. Then if a user is logged in the hidden items are then shown rather than the other way around.

Thanks for quick reply.

for example i want to hide a button from unregistered users, but it shows up fro 4-5 seconds and then hides.
i am using
document.getElementById('btn').style.visibility = 'none';

on body load

Right, exactly, you are waiting until the page fully loads until you hide the button with your page load event. You should use
Then your javascript onload event will check if the user is authenticated and display the button with:
or

You've got the answer above, but remember that people can "view source" and see things hidden from the page display...just a thought to keep in mind.

Thanks a lot for the solution
same can be used for an image or table?

Yes you can, and as langsor stated, this is not for security, if you are trying to restrict access to something you should go with a server side solution. This will only hide it from plain site, but just remember that it is not hard to view all of your javascript no matter where you are pulling it from. Even if you don't see the javascript in the source and are getting it from another file, it can be read with things like the "web developer" add on with firefox and other applications.

Any html element can be hidden/shown using this method.

Images are 'inline' elements
Table are 'block' elements

Ciao

I am using php but i haven't figured out how to hide elements based on security, can you suggest what to do?

If you are looking for a server side security mixed with the wonders of javascript then an ajax solution would be the way to go.