What is the basic legal requirement for a lawful arrest? What is the purpose of the initial appearance and the arraignment?

1- There are only a very limited number of circumstances in which an officer may make an arrest: The officer personally observed a crime; The officer has probable cause to believe that person arrested committed a crime; The officer has an arrest warrant issued by a judge. 2- An arraignment is usually a defendant's first court appearance in front of a judge and the prosecutor. The main purpose of the arraignment is to inform the defendant of the criminal charges against him or her.