While meditation is used to prevent depression relapse, meditating while in a depression or overly anxious is not recommended. It can make things worse. (I am assuming here that "meditation" means "mindfulness meditation".)

Edit:

I remember someone saying that for some very anxious people becoming aware of one's body only make them more anxious because they fear their body will fail them. I believe this is from French psychiatrist Christophe André, although I've not been able to find exact source. I'll try again later.

Another source:
http://www.wildmind.org/applied/depression/when-not-to-meditate

And this is from the Mindful Way Workbook:

> But if things are really bad right now, and your depression makes it
> just too difficult to concentrate on some of the practices, then it
> can be disheartening to struggle with new learning. It might be most
> skillful to allow yourself to wait a while if you can, or, if you do
> start, to be very gentle with yourself—remembering that the
> difficulties you experience are a direct effect of depression and
> will, sooner or later, ease.

Second edit:

It appears that currently there is anecdotal evidence that meditation can worsen symptoms, i.e. some research is needed:

> Side Effects and Risks
> 
> Meditation is considered to be safe for healthy people. There have
> been rare reports that meditation could cause or worsen symptoms in
> people who have certain psychiatric problems, but this question has
> not been fully researched. People with physical limitations may not be
> able to participate in certain meditative practices involving physical
> movement. Individuals with existing mental or physical health
> conditions should speak with their health care providers prior to
> starting a meditative practice and make their meditation instructor
> aware of their condition.

The above comes from here: http://nccam.nih.gov/health/meditation/overview.htm#sideeffects