The police can arrest you if they have reasonable grounds to suspect your involvement in a crime, either as a perpetrator or a witness. This includes situations where there's evidence of police misconduct. Being detained is a middle ground between being free and being arrested, and knowing your rights is crucial. If detained, you should inquire if you are free to leave and the reason for your detention. Remember, anything you say can be used against you in court, making it essential to consult a lawyer before engaging in detailed conversations with police.