reading multibyte characters

[Vladimir Agafonkin]

To read, for example, 20 bytes from a file, I can use the following
code:

block = input_file.read( 20 )

How can I do the same with multibyte characters instead of bytes? E.g.
I have a UTF8-encoded file, and want to read exactly 20 characters from
it, do something with them, then read another 20 character etc. Is it
possible?

Thanks in advance.