Technically, you're under arrest when you're no longer free to walk away from the arresting officer, who is obliged to inform you that you're under arrest. According to the Fourth Amendment, an arrest is only lawful if the arresting officer can show that there was probable cause to arrest you. This means that he or she had good reason to suspect that a crime took place and that you committed it. Mass arrests and confinement based on an officer's guess or unfounded suspicions are unconstitutional. If the police intend to use in court any statements made by you, they must first read you a so-called 'Miranda warning' informing you that you have the right to remain silent, that anything you say may be used against you, that you have the right to an attorney, and that you have the right to terminate the interview at any time. If you're arrested, you have the right to speak with an attorney. If you can't afford an attorney, you'll be provided with a public defender. A prosecutor must file charges against you within 72 hours, or in some states even sooner. You must be informed of the charges filed against you, and your case must be presented to a judge at an initial hearing or arraignment within a reasonable amount of time.