how to do a Password protection for a RewriteRule based VirtualHost

[KK CHN <kkchn.in@xxxxxxxxx>]

<VirtualHost *:80>
ServerName  mydomain.com
RewriteEngine On
RewriteRule ^/(.*) http://127.0.0.1:8081/VirtualHostBase/http/www.mydomain.com:80/site1/VirtualHostRoot/$1 [L,P]
ErrorLog /var/log/apache/mysite.com/error_log
CustomLog /var/log/apache/mysite.com/access.log combined
</VirtualHost>



Now  every one can access the site   mydomain.com 


But I want to restrict this access  to    a  group of members  ..  when they type www.mydomain.com   

it should   promt for a username and password  ..


I know I can do it  for  directories  but is it possible for   RewriteRule section ?  How can we do it ?


Any hints & suggestions most welcome 

Thanks in advance 
KK