The US city of Baltimore has declared a state of emergency amid violent protests over the death of a black man fatally injured in police custody.

A week-long curfew has been announced and as many as 5,000 National Guard troops could be deployed.

After dark, a community building that was under construction was engulfed in flames.

Earlier, dozens of people were seen confronting police, and 15 officers were injured.

African American Freddie Gray, 25, died on 19 April, after a week in a coma. The justice department is investigating exactly where and when his spinal injuries were sustained.

Officials have suspended six police officers who were involved in the case.

Monday's clashes began hours after Gray's funeral.

Baltimore Mayor Stephanie Rawlings-Blake said it was very clear there was a difference between the "peaceful protests of those who seek justice" and the "thugs who want to incite violence".

Authorities were "deploying every resource possible to regain control of the situation", she said.

She has ordered a week-long curfew from 22:00 to 05:00 starting on Tuesday.

National Guard commander Linda Singh said that up to 5,000 troops could be put on the streets.

"We will be out in massive force," she said, adding that armoured vehicles would be used but the city would not be under martial law.

Earlier, President Barack Obama said his administration would provide whatever assistance was needed.

Governor Larry Hogan issued an executive statement to "protect the lives and property of citizens being impacted by this public emergency".

Capt Eric Kowalczyk of Baltimore Police said the 15 injured officers were wounded by thrown objects including bricks and bottles. Two remain in hospital.

Police made at least 27 arrests.

On Monday night fire broke out at a newly constructed community building near a church in the east of the city.

Kevin Harris, a spokesman for the mayor, said the fire was related to the riots and the building is the Mary Harvin Transformation Centre, described online as a community-based organization that supports young people and families.

Before nightfall, scenes included a blaze at a CVS pharmacy, stores being looted and police vehicles being burned and vandalised.

Baltimore's Major League Baseball team the Orioles cancelled Monday night's game against Chicago's White Sox as a result of the violence.

Freddie Gray's death is the latest in a string of high-profile cases where black men have died after contact with the police.

Nationwide protests followed the killing of unarmed black teenager Michael Brown by a white police officer in Ferguson, Missouri, last year.

Baltimore officials likened Monday's violence to scenes witnessed after the 1968 assassination of black civil rights leader Dr Martin Luther King and said the city had taken a long time to recover from it, urging calm.

Earlier in the day, a statement by the police said that its Criminal Intelligence Unit learned "that members of various gangs including the Black Guerrilla Family, Bloods, and Crips had entered into a partnership" to target officers.

The violence is an escalation of earlier protests. There were 34 arrests as peaceful demonstrations by about 1,200 people outside City Hall on Saturday afternoon got out of control.

Six police officers sustained minor injuries in that episode.