I downloaded symfony2 and I am able to run it starting from app_dev.php.
But when I start from app.php, then I get an error page 404.
app.php though is of co
Your problem is related to the cache of Symfony2. I had the same problems. Try this:
sudo php app/console cache:clear --env=prod --no-debug
sudo chmod 777 -R ../sf2/
You're done.