Baltimore - Armed National Guard troopers in fatigues deployed in force in Baltimore on Tuesday after a riot triggered by anger over the death of a young African American while in police custody.
Thousands of military and police reinforcements have been ordered to the US port city to quell unrest in which cars were torched, stores looted and officers attacked.
Maryland Governor Larry Hogan was on the city streets, inspecting a National Guard barricade and vowing to "make sure what happened last night in Baltimore City is not going to happen again."
"This violence is not going to be tolerated," Hogan told reporters at the temporary checkpoint, manned by riot police and Guard troopers with assault rifles.
"By tonight, you're going to see an overwhelming display of people out there on the streets protecting the citizens," he vowed, in a nod to criticism that authorities were slow to act.
Baltimore has declared an overnight curfew from 10:00 pm Tuesday (0200 GMT Wednesday), and local and national leaders have appealed for calm after the latest in a series of incidents to stir US racial tensions.
Rioting erupted Monday after the funeral of Freddie Gray, a 25-year-old Baltimore man who died of severe spinal injuries eight days after he was detained by city police.
An investigation has been launched into the cause of his injuries, but many local people see the incident as only the latest example of police brutality against black suspects.
Gray's family and local community leaders appealed for calm, but tempers boiled over as young, mainly African American men pelted police with stones and ransacked businesses.
At least 15 police officers were injured overnight, six of them seriously, and at least 200 suspects were arrested, according to US media, citing city authorities.
"This is one of our darkest days. But I am confident that the will of good can overcome the destructive interest of a few," city mayor Stephanie Rawlings-Blake said on Twitter.
AFP