This armored truck allows police to shoot pepper spray while driving through protests

Police and National Guard members are struggling to calm protesters in Baltimore as riots grip the city.

Pepper spray is a common tool that's seen in such riots - it was also ubiquitous in Ferguson, Missouri during the protests there last summer - as authorities try to subdue crowds and bring them under control.

An AP photo shows how one armored truck allows police to spray protesters from inside their vehicle:

AP

Baltimore police officer sprays pepper spray to demonstrators after the funeral of Freddie Gray, Monday, April 27, 2015, at New Shiloh Baptist Church in Baltimore.

The Baltimore riots that broke out on Monday have been dangerous for police. More than 15 cops were injured as protesters pelted them with bricks, rocks, and other objects.

Police have used tear gas as they fight back against the crowds. Many cops are in full riot gear, which includes shields, but that hasn't always been enough to protect them.

Sait Serkan Gurbuz/Reuters

Sergeant C.J. Warren, who said he was hit by a brick during clashes with protesters near Mondawmin Mall after Freddie Gray's funeral, is pictured in Baltimore April 27, 2015.

REUTERS/Shannon Stapleton

A demonstrator looks up after being sprayed with pepper spray during clashes in Baltimore, Maryland April 27, 2015.

Chip Somodevilla/Getty Images

Police fire tear gas canisters as violent protests move along Pennsylvania Avenue following the funeral of Freddie Gray April 27, 2015 in Baltimore, Maryland.

At least 27 people have been arrested in the protests, which started with a confrontation between cops and teenagers outside the Mondawmin Mall kicked off the riots in Baltimore.

The rioting follows the funeral of Freddie Gray, a 25-year-old black man who suffered a spinal-cord injury and died while in police custody. Protests started shortly after Gray died but grew out of control on Monday.

Baltimore police arrested Gray "without force or incident" on April 12 on suspicion of possession of a switchblade knife. While in police custody, Gray suffered a "medical emergency" that severed his spine 80% at his neck, according to a statement from his family attorney, William "Billy" Murphy Jr.

It's still unclear exactly what happened to cause the injury.

Getty Images/Chip Somodevilla