You really should not do this, you would be better off redirecting to a holding page and link to your Index Page from there, as otherwise you run the risk of getting delisted from search engines like google.
I know this thread is a few months old, but I can use a good line for my htaccsss to direct old pages to the index without issues for my SE placement. I'm revamping some sites with mega-content and need a simple solution. As well for some of my newer domains which get historic traffic from SE, I'd like to capture that traffic.
The line mentioned by Daniel works perfectly and is as simple as it gets, but I'm concerned with what namestrands states.
Jack.. Just create a new page 404.htm and redirect all old non existent pages to this.
Then have a link to your home page. This way you will not damage your index page in the search engines.. redirecting many pages to your index page will cause duplicate content and you will eventually get delisted or at least put in the sandbox.
However, if the old pages have equivilent new pages then consider doing a 301 redirect, this will maintain any page rank the old pages have and will pass it on to the new ones.