An arrest is the act of legally restraining an individual. In a legal capacity, arrest implies that there is a level of entitlement or obligation to do so. A police officer is instilled with the legal authority to arrest a person when said individual is believed to be in violation of the law. Any criminal action that is reported to the police can initiate the arrest of a suspect for further interview or in order to put the person in front of a judge to determine guilt or innocence. Arrests can also be made for safety reasons.
In order for an arrest to be deemed legal, it must be done either by a person entrusted under law to monitor and prevent crime (i.e. a police officer) or by a person or group who does so for the sole purpose of detaining a person until the police arrive. In any case, the person under arrest has to be detained in a legal manner: without any more restraint than is necessary. There are also several other procedures that law enforcement officers must follow during an arrest, such as the reading of Miranda rights.