An arrest occurs when a police officer takes away a person’s freedom to walk away from an encounter with the police. In the eyes of the law, seizure of a person’s body has effectively occurred, and therefore arrests are covered by the Fourth Amendment protections from unreasonable seizures.
Police officers are not justified in making an arrest unless they have probable cause, a reasonable suspicion, to believe that the suspect committed a crime. In order to establish probable cause, the officer must have objective facts.
A police officer might make an arrest with an arrest warrant in hand. This means the officer has gone before a judge with enough persuasive factual evidence to implicate the suspect. Typically, the arrest warrant lists the crime and the name of the person to be arrested. It may also specify the time and place where the arrest can be made.
Generally, police officers are authorized to make arrests without a warrant, as long as they have probable cause to believe a crime was committed. This is called a “warrantless arrest.” Later, it is up to a judge to decide whether the police indeed had probable cause. In this case, the judge is required to make a speedy determination. If it is determined that the police did not have probable cause, the suspect will be released and any evidence discovered as a result of that arrest will not be admissible at trial.
Example: You are arrested merely because you are standing outside a liquor store that was just robbed. If the officers have no basis for arresting you–i.e., you don’t fit the description of the robber, or you are not acting suspiciously–then they have arrested you without probable cause and you may be free to leave.