Despite what many people think, the concept of citizen's arrest is not to circumvent legitimate law enforcement or arbitrarily detain people without proof of a crime. Today, it exists as more of an emergency or stop-gap power granted to ordinary citizens at the behest of law enforcement officers.
Citizen's arrest means that a private citizen has the right to detain suspected criminals until proper law enforcement personnel can assume custody. The practice can be traced back to English common law during the Middle Ages, although each country or state can modify the rules of engagement. During the earliest days of modern justice systems, performing a citizen's arrest was a much more common practice. Merchants would routinely detain shoplifters and thieves caught in the act, often bringing them directly to a local constable's office for trial. As criminals became better armed and law enforcement became more readily available, the popularity of this type o arrest seemed to wane.
Modern law enforcement officers strongly discourage untrained civilians from making a citizen's arrest. The risk of bodily injury or death is much too high, and the average response time of trained police officers is significantly faster. But under certain circumstances, such an arrest can provide enough time for the proper authorities to arrive. One of the main problems with a citizen's arrest, however, is the possibility of making a mistake. Unlike a failed attempt to resuscitate a victim through CPR, there is very little if any 'Good Samaritan' protection for private citizens who detain an innocent suspect.
One criteria for a legal citizen's arrest is the immediacy of the crime. The ideal circumstance is to catch the suspected criminal in the very act of committing a crime. A person witnessing a mugging can seize the mugger and hold him until a police officer arrives, for example.
Another scenario for a proper citizen's arrest would be a serious potential for a crime to be committed soon. If a person saw a masked man with a crowbar walking towards a vehicle, it can be reasonably assumed that a crime is about to take place. The witness can detain the masked man. This would still be true even if the 'crime' turned out to be a misunderstanding. If someone sees a man climbing through a broken window, he cannot be held responsible for false arrest if the man turns out to be the owner of the building who lost his keys.
Because the act of detaining an armed or physically powerful suspect can be extremely dangerous, police officers often suggest that citizens spend their time observing the suspect and the crime scene instead. If the witness can provide a physical description of the suspect or a license plate number, the police may be able to find the suspect themselves. The safer equivalent is a signed statement, with the intention of pressing criminal charges later. Sometimes, a police officer will ask a witness or victim to tell the suspect that he or she has been placed under citizen's arrest. This gives the police more legal authority to detain the suspect until he or she can be properly processed into the legal system.
