Some golden rules:

Do NOT admit responsibility or sign an accident report form presented by the other driver. Inform your insurance company with 24 hours and send them outline details of the circumstances, the location, the details of the other driver and the names of the witnesses.

Leaving the scene of a serious accident is a grave offence in Spain. You should always wait, even if you are only a witness, until the Police arrive. If you are at fault and you are detained by the Police, you must ask for assistance of a translator or call your Solicitor, who will advise you not to sign a statement until the details have been explained, or better still, until he can have someone from a competent legal firm visit the Police Station to assist you