from: http://ukinspain.fco.gov.uk/en/13821078/legal-advice

Background

1. The Spanish legal system is different from that of the United Kingdom although, as in Britain, the Judiciary is independent of the Executive and all foreigners, like Spaniards, have rights guaranteed under article 17 of the Spanish Constitution. See Annex A.

2. An offence normally first comes to the notice of the police when the injured party makes a report or complaint (denuncia) or when the police are called to the scene of a reported crime. However, in some instances (e.g. as a result of enquiries or traffic offences) the police themselves make the initial complaint (atestado policial). Under the terms of the UK-Spain Bilateral Convention the Police should inform the nearest British Consulate of the arrest of a British Citizen if he/she agrees to them doing so. A consular officer will then offer consular assistance to the detainee as soon as possible after receiving notification of his/her arrest.
Arrest

3. Upon arrest a person should be informed of the reason for his/her arrest and of his/her rights. If they have insufficient knowledge of Spanish an interpreter should be insisted upon. He or she is then likely to be held temporarily in a police cell until a formal statement answering the charges can be taken. After this, but in any event within 72 hours of arrest, (24 hrs in case of minors) the accused will be brought before a Judge (Juez de Instrucción - Examining Magistrate) or released. A lawyer and, if necessary, an interpreter (if you speak sufficient Spanish to understand basic legal terminology you could waive your right to an interpreter) must be present when the statement is taken and when the accused appears before the Judge. If the accused is unable to pay for, or doesn´t know of one, a lawyer will be assigned to him or her at no charge (although a bill may be presented later if it transpires that the accused has means to pay) from a duty roster of the local Bar Association (Colegio de Abogados). But it is important to note that, at this stage, the lawyer is present solely to ensure that the accused's Constitutional rights are guaranteed and that the correct legal procedures are followed. He is not there to "defend" the prisoner against the substantive allegations or help the accused to prepare a declaration before the police. He/she will be allowed, however, to speak privately with the lawyer after declaring or declining to declare before the Police.

4. In certain very special circumstances (primarily charges concerning terrorism or major drug trafficking) a prisoner can be held incommunicado for up to 72 hours. Also in exceptional cases a Judge can extend the 72-hour period by up to another 2 days. (Article 384 of the Criminal Prosecution Act).
5. When appearing before the Examining Magistrate the accused will be asked to confirm or deny any statement that may have previously been made to the police or to make a further statement. This initial appearance is a brief oral examination of the accused by the Judge and in the presence of the Public Prosecutor and will normally be based on evidence submitted by the police. From this he will determine whether there is a case to answer and, if so, decide whether the accused should be released without charges, placed on provisional liberty with or without bail (fianza), or remanded in custody (prisión preventiva) pending further investigation. The decision by the Examining Magistrate is subject to appeal by both the defendant and the prosecutor (Fiscal).

Remand in custody (Prisión preventiva) Articles 503,504 LEC

6. There is a maximum period during which an accused person can be held in prison without trial. This could be up to 12 months where the alleged offence is punishable by a term of imprisonment of three years or less, and up to 24 months where it would be punishable by a term of three years and one day or more. But if the investigating Judge is satisfied that there are valid reasons for the police not being able to complete their case against the accused within the time, or there is reason to believe that the accused will escape the Court's jurisdiction if released, the Judge may extend the pre-trial detention period by a further 6 and 24 months respectively. This means in effect that a person can be held on remand up to 4 years. If a sentence is passed and appealed against, the accused can be held waiting appeal up to half the sentence imposed. If the appeal has not been heard by this time the accused must be released pending the result of the appeal.
Structure of the proceedings
Investigation phase

7. The Examining Magistrate is in charge of investigating the crime, its circumstances, perpetrators and any other matters relating to the offence. He is assisted in this by the judicial police (members of the national police force assigned to his office). The State Prosecutor, defence lawyer (and, if appointed, private prosecuting counsel) may ask the Judge to follow specific leads in the investigation. The Prosecutor is at the same time charged with the legal duty of ensuring the defendant's rights are respected and the victim's rights protected. When the investigation is completed, if there is a prima facie case, it is the prosecutor (Fiscal) who lays the formal accusation based on the evidence and he will eventually present the case for the prosecution in court. During the investigation stage all evidence, including police documents and witnesses' statements, are reserved documents and copies will not normally be released to interested parties or their representatives although, of course, defence lawyers and, if appointed, private prosecuting lawyers do have the right of access and examination. A judge can, nevertheless, in exceptional cases, e.g. those involving state security, severely restrict the access to papers, stating that the investigation shall be declared secret.(Secreto de Sumario)
Intermediate phase

8. Once the investigation activities have been carried out, the Examining Magistrate will end the investigation phase and an intermediate phase before the eventual trial will start. For this purpose the Examining Magistrate will issue one of the following orders:
i) declaring that the offence has not been committed or that the identification of the culprits was impossible and ordering the closure of the file;
ii) declaring that there is enough circumstantial evidence of the commission of the crime and the identification of the culprits to proceed. This order may be appealed, although the appeal does not suspend proceedings. In the latter case, the Examining Magistrate will grant a five day term to the Public Prosecutor and to the private accusation and the defence, in which they can submit one of the following Briefs i) that they wish to continue with the proceedings (accusation writ): they accuse and propose the evidence steps to be carried out during the main hearing); ii) that they do not wish to continue with the proceedings (a dismissal brief) or iii) that they think it is necessary to carry out additional investigations before taking this decision.

9. The Examining Magistrate ends this phase and issues one of the following orders:
i) if no facts of a criminal nature result from the investigation: closing and filing the proceedings with a discontinuance order; or
ii) transfer of the file to the Trial Court if there is enough evidence of the possible existence of a criminal offence.

This order can only be appealed if the Magistrate decides to close and shelve the proceedings. Should the Public Prosecutor or the private accusation file an accusation brief, the Examining Magistrate will most probably issue an order of type ii), i.e. ordering that the proceedings should continue. In this case, the accusing parties are granted a ten-day term to file their accusation writs and propose the means of evidence for the trial. The defendant is also given a ten-day period to determine its position on the accusation, and propose means of evidence for the trial. If not already carried out, The Examining Court can also require the accused to give a financial bond in order to secure possible future civil liabilities.

In certain cases where the alleged crime is punishable by less than 6 years’ imprisonment, it is possible for a defendant to admit guilt by signing a formal brief stating that he/she is in agreement with the Prosecutor’s allegations.