symfony2 - how to switch from "dev" to "prod"?

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 course there and it gets executed.

The error happens apparently somewhere after the last line of code in app.php:

$kernel->handle(Request::createFromGlobals())->send();

I guess there is a switch somewhere I have to configure.

EDIT:

As suggested by GeLo I added the routing for the production version to app/config/routing.yml.

_welcome:
    resource: "@AcmeDemoBundle/Resources/config/routing.yml"
    prefix:     /

I created src/Acme/DemoBundle/Resources/config/routing.yml with content:

bla:
    pattern: /
    defaults: {_controller:AcmeDemoBundle:Demo:index}

In DemoController::indexAction() I placed a die(__FILE__);

nothing; I still get 404 from app.php !?

EDIT regarding the answer:

./app/console --env=prod cache:clear

did the trick. mind the env-parameter.