The Friendly url rules can do that :
Match : .*/portal/(.*) -> Replace : ~/$1
This is a rewrite rule.
The result of the replace have return a internal url (Default.aspx?TabId=.....)
This work if your old urls are /portals/Default.aspx?tabid=...
If your old urls are /portals/page/tabid/...../default.aspx
You can use
Match : .*/portal/.*/TabId/(\d+)(.*) -> Replace : ~/Default.aspx?TabId=$1
Know also that the new and the old url return the same content (bad for seo)
Other solution is a redirect with the friendly rules :
Match : .*/portal/(.*) -> Replace : http://www.mysite.com/$1
You can also use IIS rewrite module (free with iis) which is more flexible.