Formal:
* Por favor, faça silêncio. (Please, make silence.) - This is the most formal option.
* Por favor, seja silencioso. (Please, be silent.) - This is also a formal option, but slightly less so than the first.
Informal:
* Por favor, cala-te. (Please, shut up.) - This is very informal and can be considered rude depending on the situation and the relationship between the speaker and the listener.
* Fique quieto, por favor. (Be quiet, please.) - This is more polite than "cala-te" but still informal.
* Silencio, por favor. (Silence, please.) - This is a more direct way to ask for quiet, but it is still polite.
Other options:
* Pode fazer silêncio, por favor? (Could you please be quiet?) - This is a polite way to ask for quiet, and it is appropriate for both formal and informal situations.
* Você poderia fazer silêncio, por favor? (Could you please be quiet?) - This is another polite way to ask for quiet, and it is more formal than "Pode fazer silêncio, por favor?".
The best way to say "please be quiet" in Portuguese depends on the specific situation and the relationship between the speaker and the listener.