Find and replace in visual area.

[Silva, Paulo]

Greetings,
I'm trying to do a replace in a selection.
After selecting the area, with v, directional keys, v again (or not -
both give the same result).
 
Then I type
:%s/\%V20/21/
 
and I get:
E71: Invalid character after \%
 
This works the same with any caracter that I put after the \% even with
no character.
 
Am I dooing something wrong?
Thanks