But critics complain that the tactic comes with an unjust catch: If vandalism or rock throwing begin, officers order crowds to disperse and block all exits. The result, they say, is an arrest net that sweeps up journalists, bystanders and peaceful demonstrators.
In Ferguson, police drew heavy criticism for their militarized response to protests after unarmed 18-year-old Michael Brown, who was black, was fatally shot on Aug. 9, 2014, by white officer Darren Wilson, who was not charged and later resigned.
When the trouble begins, additional officers quickly arrive from staging areas, with shields, helmets and gas masks.