Updated : 26/04/2017
EU citizens have the right to stay in Portugal for up to 3 months. The only condition is that they must hold a valid identity document (an identity card, a card proving citizenship or a passport).
The same principle applies to the family members of an EU citizen who hold a valid passport and are accompanying that person or coming to Portugal to be reunited with him or her.
EU citizens do not have to register during their first 3 months in Portugal (to obtain a document proving their right to be in the country), but they may do so at the local town hall (Câmara Municipal) if they so wish.