Silex / Symfony login form

I have Silex login form factory with not blank,length validation constraints for fields (username & password).

I need to validate this fields after submit form and before authentication starts. If validation on fields fails, then redirect to login page with input errors, else starts authentication and give its response about creditions (if fails).

I don't find any answer on my question in Symfony2 and Silex documentations, there are examples with Http Basic.

Can you give me any ideas?