The Federal Building, on Liberty Street between Fourth Avenue and Fifth Avenue, houses the downtown Post Office and a variety of federal government offices, including a portion of the United States District Court for the Eastern District of Michigan.
There is a plaza out front which is often the starting point for parades downtown and for rallies and political actions. Protesters congregate there every weekday from 5-6pm. You can bring your own sign or use one of their pre-printed set.