A police officer may arrest you if (1) the officer observes you committing a crime; (2) the officer has probable cause to believe that a crime has been committed by you; or (3) the officer makes the arrest under the authority of a valid arrest warrant.
Generally if a police officer has probable cause to believe that a crime has been committed, the officer may make an arrest without an arrest warrant.
The police can stop you, and ask questions, without “arresting” you. Upon seeing suspicious activity, the police may perform what is called a “Terry Stop,” and may temporarily detain people to request that they identify themselves and to question them about the suspicious activity. The scope of a “Terry Stop” is limited to investigation of the specific suspicious activity, and if the police detain you to question about additional matters, the stop can turn into an “arrest.”
For their own safety, the police can frisk you and if they feel something that may be a weapon, they may remove it from you for further examination. However, they are not entitled to remove items from your pockets that do not appear to be weapons, even if they believe that the items are contraband