there are many files of avast in my temp file directory . are they safe to delete they are taking much space
Sure,you can delete everything inside TEMP folder,just exclude avast! folder,because there is one avast! system file (Virus Cleaner) 
You can delete the whole folder, that file is used for memory-scanning during avast’s starting.
And it serves as integrated Virus Cleaner. If you delete that file,you’ll get error if you try to run it using GUI 
what is gui ?
Graphic User Interface or in short,menus and buttons you see allaround
Not true, I’m afraid. The code for startup scan is integrated directly in one of avast! DLLs. This particular file is used either when the user selects “Start Virus Cleaner” from the Enhanced User Interface, or from the Virus Dialog when the resident protection finds a virus.
And what did i sayd
And it serves as integrated Virus Cleaner. If you delete that file,you'll get error if you try to run it using GUI
I just corrected the “And” word from your post ;D