Freddie Gray: Gangs make 'credible threat' to police
Baltimore police say they have received "credible information" that members of various - sometimes rival - gangs are partnering to "take out" officers.
The information was released during Monday's funeral service for Freddie Gray, which was followed by more street clashes between protesters and police.
Gray, 25, died a week after sustaining serious and unexplained spinal injuries during his arrest.
His death sparked daily protests about police force against African Americans.
On Saturday the protests turned violent, with some elements confronting police officers and smashing cars.
A statement released by police said that its Criminal Intelligence Unit learned "that members of various gangs including the Black Guerrilla Family (BGF), Bloods, and Crips have entered into a partnership" to target officers.
Some protesters had warned on Saturday that the gangs would come together in this way, says the BBC's Aleem Maqbool.
One man told him: "We gonna unify, you got Bloods, Crips, Muslims, BGF, you got everybody out here, you ain't never see it like this before. Everybody together just to go against these pigs [police].
Meanwhile, around 2,500 people are reported to have packed into the New Shiloh Baptist Church to pay their respects to Gray.
President Barack Obama has dispatched three officials to the service.
Analysis - Aleem Maqbool, BBC News
There have been some large, very peaceful protests in Baltimore since the death of Freddie Gray, attended by hundreds upset by what they perceive to be years of police discrimination and brutality.
But in many respects they have been overshadowed by more violent displays by small numbers of angry young protesters.
Over the weekend, as we watched young men hurling abuse, bottles and even spitting at police officers, we saw other demonstrators try to stop them and argue that only peaceful means would work.
But those confronting the police told us they felt the only way to stop officers killing black men was to "make the police pay" and to harm them.
They said the various gangs in the city would come together to make that happen until they saw a change, and felt they had got justice.
Elijah Cummings, a member of the US House of Representatives, was one of the speakers.
Looking at the heavy media presence, he said: "I'm used to a lot of cameras, I ain't seen this many cameras in a long time.
"I put my nephew in the grave four years ago - blasted away, still don't know who did it," he told the crowd as he fought back tears.
"We will not rest until we address this and see that justice is done," he said referring to Gray's case.
On Sunday afternoon, mourners - many of whom did not know Gray - streamed into the funeral home for his wake.
Some stood outside holding signs emblazoned with "We remember Freddie" and "Our Hearts Are With The Gray Family".
The same day, Baltimore Mayor Stephanie Rawlings-Blake appealed for peace after protests the night before turned violent.
There were 34 arrests as peaceful demonstrations by about 1,200 people outside City Hall on Saturday afternoon got out of control.
Small groups and individuals splintered from the main group, looting a storefront, tossing a flaming rubbish bin at police, and smashing police cars.
Six police officers sustained minor injuries.
The protests, the largest to date, were just one episode of near-daily demonstrations that have taken place since Gray's death.
Gray lay in a coma for one week before his death, a week after police chased him through a West Baltimore neighbourhood and dragged him into a police van.
Police say they arrested him after he made eye contact with officers and ran away.
Officials have already accepted that procedures were not properly followed during the arrest.
During the arrest and van ride, Gray asked for medical attention a number of times, but was refused.
After a 30-minute ride with several stops, including one to place Gray in leg shackles, paramedics were called.
Authorities have so far not explained when or how Gray's spine was injured.