BALTIMORE (REUTERS/AFP) - Seven Baltimore police officers were injured, some seriously, as rioters threw bricks and stones and burned patrol cars in violent protests after the funeral of a black man who died in police custody.

The riots broke out Monday just a few blocks from the site of the funeral of 25-year-old Freddie Gray in northwest Baltimore and then spread through other parts of the city, after local law enforcement warned of a threat by gangs.

Television images showed looting and a mob of rioters jumping on the top of a police car, after teenaged crowds ignored calls to disperse and clashed with a line of hundreds of police.

Gray's death reignited a public outcry over police treatment of African-Americans that flared last year after the killings of unarmed black men in Ferguson, Missouri, New York City, Cleveland, and elsewhere.

The rioting in Baltimore was the most violent since the demonstrations in Ferguson last year.

Baltimore Police Captain Eric Kowalczyk said at a police briefing that one of the injured police officers was unresponsive and several had broken bones.

Kowalczyk said police, who initially tried to use restraint, would begin making arrests and using tear gas and pepper pellets to break up crowds.

Gray's family, pastors and city officials had pleaded for peaceful demonstrations after some arrests and injuries at protests over the weekend.

Earlier, at the funeral, speaker after speaker before the crowd packing the 2,500-seat New Shiloh Baptist Church said the world was watching to see if justice would be done for Gray, who died on April 19 from a spinal injury after police arrested him a week before.

"We hope that it doesn't get any worse than that, frankly, because Freddie Gray's family does not want this," Bill Murphy, attorney for Gray's family, told CNN in an interview.

The violence followed both Gray's funeral and a message on social media announcing a "purge" - street slang for random acts of lawlessness - when schools let out for the day.

Fear of unrest prompted the University of Maryland's downtown campus and several businesses to shut down early.

Gray's death sparked heated demonstrations over the weekend in the blue-collar port city, and police said some 34 people were arrested and six officers injured in street violence.

Police meanwhile announced they had received a "credible threat" that several known criminal gangs in Baltimore had "entered into a partnership to 'take out' law enforcement officers."

Friends, family and strangers came together Monday to bury Gray, who lay in a 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 was displayed among floral wreaths.

"I'm here to pay my respects," said Kenny Nicholson, a friend of Gray's who attended with his wife.

Supporters, many dressed in all white, filled the building's 2,200 seats and hundreds of others stood, with the words "Black lives matter and all lives matter" projected on the wall.

Civil rights activist Jesse Jackson denounced the "epidemic of murders in the country." "We have become too violent, too full of hate," Jackson told reporters before the service.

"We need training, employment, housing, access to health, a reconstruction project. Poverty is a weapon of mass destruction." Gray's family lawyer echoed calls for reform, and called for police across the country to wear body cameras to capture confrontations with suspects.

"We are here because of Freddie Gray, but we are here because there are a lot of Freddie Grays," attorney William Murphy said at the service.

"There is a corrosion of justice around here," he added, calling for the appointment of a special prosecutor to look into Gray's death.

White House cabinet secretary Broderick Johnson was at the service, according to a senior administration official.

Heather Foster, an advisor in the White House office of public engagement was also present, along with Congressman Elijah Cummings, who gave an emotional address to the crowded room.

Gray's death sparked days of protests last week, and turned violent late Saturday.

Baltimore Mayor Stephanie Rawlings-Blake, who attended Gray's funeral, had called for calm Sunday.

"I hope that as the eyes of the country are on Baltimore, that we see very clearly that this is a community that's willing to confront tough issues, that's willing to demand accountability, but also demands peace and progress at the same time," she said, according to the Baltimore Sun.

Tensions have risen in Baltimore since Gray's death, which lawyers said was caused when 80 percent of his spine was severed following his arrest.

Authorities have launched an investigation into the incident, and six officers have been suspended with pay.

Police confirmed Gray requested medical help and an inhaler after he was detained and said he should have received medical attention sooner.

They also revealed that Gray, contrary to policy, was not buckled into his seat in the van, which made at least three unexplained stops on its way to the police station.

Gray's arrest was caught on video by bystanders, and he can be heard howling in apparent pain as his limp body is dragged by police.