D.C. Code § 5-331.08
No emergency area or zone will be established by using a police line to encircle, or substantially encircle, a demonstration, rally, parade, march, picket line, or other similar assembly (or subpart thereof) conducted for the purpose of persons expressing their political, social, or religious views except where there is probable cause to believe that a significant number or percentage of the persons located in the area or zone have committed unlawful acts (other than failure to have an approved assembly plan) and the police have the ability to identify those individuals and have decided to arrest them; provided, that this section does not prohibit the use of a police line to encircle an assembly for the safety of the demonstrators.
D.C. Code § 5-331.08