UPDATED 1.23PM: The governor of the US state of Maryland has declared a state of emergency in the capital Baltimore and will deploy the National Guard as soon as possible, the city's mayor says, as riots continue.

PHOTOS: Riots erupt in US city

"Too many people have spent generations building up this city for it to be destroyed by thugs," Mayor Stephanie Rawlings-Blake said.

Rioters smashed police vehicles, looted stores and pelted police with rocks on Monday after the funeral of Freddie Gray, 25, an African American man whose death in custody has reignited outrage over US police conduct towards blacks.

Rawlings-Blake told a press conference that Governor Larry Hogan had agreed to deploy National Guardsmen "as soon as they are available".

In the meantime the the port city of 620,000 will be placed under curfew from 10pm on Tuesday (1200 AEST Wednesday).

"This preliminary curfew will last for one week and be extended as it is necessary," Rawlings-Blake told reporters, flanked by her police chiefs and local leaders.

Police said 15 officers have been injured and two remained hospitalised, The Associated Press reported. About two dozen protesters had been arrested.

Local and state police in riot gear struggled to restore order as the rioters, believed to be mainly teenagers, veered off in different directions, refusing to heed dispersal orders.

Rioting erupted soon after Gray was buried - possibly spurred by a cryptic message on social media declaring an after-school "purge", which is street slang for random acts of lawlessness.

Fear of unrest prompted the University of Maryland's downtown campus, corporate offices and the city's famous Lexington Market to shut down early.

President Barack Obama was briefed on the rapidly evolving situation by his newly sworn in Attorney-General Loretta Lynch and Rawlings-Blake, the White House said.

Thousands had converged on New Shiloh Baptist church in Baltimore's poverty-ridden Sandtown neighbourhood earlier on Monday to pay final respects to Gray, who died on April 19 of severe spinal injuries apparently sustained during his arrest a week earlier.
Gray's grieving family had explicitly asked for no protests.

"Today of all days, the family was clear this was a day of sacred closure," pastor Jamal Bryant of the city's Empowerment Temple mega-church, who delivered the eulogy, told reporters as the violence spiralled.

"So for us to come out of the burial and walk into this is absolutely inexcusable. I'm asking every young person to go back home."

At the funeral, Gray's body was in a white casket next to a Los Angeles Dodgers baseball cap and a sign reading "Peace y'all".

Crowds swayed to hymns at the service, chanting, "Justice shall prevail, peace will prevail" in the church, where a photo of Gray - who had a record of petty drug offences, in a grim part of Baltimore notorious for crime, poverty and joblessness - was displayed among floral wreaths.

Tensions have been on the rise in Baltimore since Gray's death, which his family's lawyers say was caused when 80 per cent of his spine was severed following his arrest.

Six officers have been suspended with pay pending the outcome of a police investigation that is to be submitted to state prosecutors by Friday.