The structure and organisation of the Fuerzas y Cuerpos de Seguridad (Security Forces and Corps) is complex, so take this as a rough and ready guide rather than the definitive truth. I've omitted the blood soaked past and avoided any sort of critique so I apologise now if the post is a bit dry and dusty.
Nationally there are two police forces. The Guardia Civil and the Cuerpo Nacional de Policía (CNP). In very broad stroke the CNP operate in urban areas and the Guardia Civil operate in rural areas. The Guardia Civil use a green and white colour scheme while the CNP uses blue and white. Both add the Spanish flag into their livery. Both these national police forces are controlled by the Interior Ministry but the Guardia is a military unit with responsibilities to the Ministry of Defence.
In any town with more that 5,000 inhabitants there has to be a police force employed by the local Town Hall, these are Local Police or Policia Local. In Madrid they are called Policía Municipal and, in Barcelona, Guardia Urbana.
All the police forces do those things that you expect from European police - protecting people, goods and property, maintaining order, preventing crime, investigating offences and collecting intelligence about potential crime. The two national forces have different, but sometimes, overlapping responsibilities.
The CNP (68,000 officers) works in the provincial capitals and in specified urban centres; as a rule of thumb those with more than 30,000 inhabitants.The CNP issues identity documents and passports and has responsibility for combating drug crime, organised crime, cyber crime, gambling and forgery. They also do border control, immigration and human trafficking. They are responsible for coordination with international police forces and they control private security firms. Among the specialist functions the CNP has a bomb squad, a "SWAT" team, the GEOs, and more.
The Guardia Civil (78,000 officers) deals with traffic on the main roads, looks after the security of things like ports and airports, looks out for environmental crimes, moves prisoners around and issues all the gun and explosives licences. They have specialist units for anti smuggling, for tax related and economic crimes. The Guardia do mountain rescue and they patrol the southernmost frontiers of Europe in Ceuta and Melilla.
The Local Police (66,000 officers) look after the protection of local councillors and council property, deal with urban traffic, are responsible for crowd control at public events (along with Civil Protection), mediate in conflicts between neighbours and cooperate with the other police forces.
Now the exceptions. Because of Spain's almost federal structure there are a regional forces in Catalonia (Mossos d'Esquadra), the Basque Country (Ertzaintza), Navarre (Foruzaingoa) and the Canary Islands (Guanchancha). About 28,000 officers in all. The Basque and Catalan police forces do most of the jobs which are usually associated with the Guardia Civil and CNP within their regions. In Navarre and the Canary Islands the regional police forces are less "powerful" but they still have a very visible presence for day to day policing including serious crimes like murder. In Andalucía, Aragón, Asturias, Galicia and Comunidad Valenciana there are regional police forces that are part of the CNP but have a certain independence. If you live in Alicante you may have noticed the light blue and white police cars of the Valencian Police from time to time.
The title?, it was a song, If you want to know the time ask a policeman.
