Traffic police in Portugal
Portugal traffic police are the GNR (rural areas) and PSP (urban areas). Expect strict enforcement of driving laws and fines. Emergency services call 112.
Police forces in Portugal
GNR: The Republican National Guard (Guarda Nacional Republicana) is a para-military gendarmerie, not strictly a police force. Similar to the Guardia Civil in Spain, the gendarmes in France and the carabinieri in Italy. More details on Wikipedia. In Portugal they mainly have control of rural (countryside) areas. Report ultimately to the Ministry of Internal Administration and Ministry of National Defence.
PSP: The Public Security Police (Polícia de Segurança Pública) are more of a true police force with civil rather than military origins. More details on Wikipedia. In Portugal the PSP mainly has control of urban (cities, towns) areas. Report ultimately to the Ministry of Internal Administration.
Judicial police: Concentrate on serious crime. Not typically involved in traffic policing. Report ultimately to the Ministry of Justice.
Most northern European countries have distinct lines between military / police / judiciary. That is less the case in southern European countries like Portugal. The country could probably benefit from a radical simplification and integration of current police forces, but there are huge cultural and historical issues here.
Which police force you might encounter in Portugal (either PSP or GNR) depends upon where you are. In cities it is likely to be the PSP, elsewhere the GNR. If the police attend your vehicle (at an accident, when they fine you for traffic violations such as speeding etc.) you can distinguish them firstly by appearance. PSP vehicles have a blue-and-white livery and the officers wear mostly blue uniforms. GNR vehicles have a yellow-and-green livery and the officers wear mostly brown uniforms. Both GNR and PSP should enforce Portuguese law equally.
Know how to behave if you are involved in an accident while driving in Portugal. Sure you can stand up for your rights to make sure the accident details are correctly recorded. But equally a little bit of courtesy goes a long way. Avoid being rude or abrasive with traffic police officers in this country. The vast majority of officers are professional and well-trained, and the level of spoken English is generally very good. They want to enforce the law then move on. The last thing they need is a bad-tempered non-Portuguese national being foul-mouthed and abusive.