I want to remove the shower pan to replace it with ceramic tiles but it doesn't come off easily because it tightly connected to the floor. I am not sure how it's been installed: with glue? Mortar? Else? And how to remove it? Any help is greatly appreciated. It's been there for over 20 years. Thank you.

Don't you still need the shower pan ?

Well, all depends on what we call here a shower pan. What I am referring to is the rectangular porcelain tub which is now old and needs be replaced. Of course there is a shower pan that must be built or installed under tiles for preventing leaks. So, in short, I am trying to remove the above mentioned tub -- whether it's really called shower pan or base, I am not too sure... Hope it's now clear, thanks for asking.

Iraj, I'm 60 years old and have not seen a porcelain shower pan, doesn't mean they do not exist but I have never seen one. Are you sure its porcelain?

Well, may be it's not porcelain because I just noticed that I could kind of bend its outside of it. I guess then it is a PVC or some other plastic material, but whatever it is, it is a hard material... thanks.

It is most likely acrylic or fiberglass. Other than the drain assembly they are held in a bed of modified thinset and my require a prybar to break them loose from the floor.

Well, as it turned out, I could remove the tub by 1st, taking out 4 rows of tiles. The edges of tub had been tucked under the drywall under the tiles. I had to cut around the drain hole using a electric saw. The tub was laid over the plywood with no adhesive or any other material to keep it firm. So, once this was done, I could completely remove it. I got the idea from the following link:
bathroom remodeling shower pan liner (http://mrtile.com/replacing%20fiberglass%20shower%20pans.htm)
Any way, thank you for trying to help me.