I've been detained!
Remain Calm.
Ask "Am I being detained, or am I free to go?".
Calmly walk away if you are free to go.
If you are under arrest, you have a right to ask why.
Say you wish to remain silent and ask for a lawyer.
You never have to consent to a search of yourself or your belongings.
You have the right to a local phone call. If this is to your lawyer the police cannot listen in, calls to anyone else they can.