(extract from "You and the Law in Spain" by David Searl published by Santana Books - a very good reference book - highly recommended!)

If you are a European Union Citizen, you are entitled not only to vote in the municipal elections in the town where you are registered, but to be elected to office as well.  This right applies even if you do not hold a Spanish residence permit.

Go to the Town Hall with your passport or your residence permit and some evidence that you live in the municipality, such as a rental contract or a property deed, and inscribe yourself in the PADRON, the list of local residents, and receive a CERTIFICADO DE EMPADRONAMIENTO, a registry certificate for your municipality.

They can refuse if you do not spend more than six months per year residing in the locality.  This rule applied to both Spaniards and foreigners. Persons are prohibited from registering in two municipalities at the same time.

You will also be expected to vote for your EU representative in Spain and not your original country if you are registered locally.

You then request that your name appears on the CENSO ELECTORAL, the voter registration list, and you will be ready to vote in the local and EU elections.