BALTIMORE , Md. ( CNN) -- A riot erupted on the streets of Baltimore late Monday as protesters clashed with police, several of whom were injured.
CNN reports a state of emergency was issued and the National Guard was called in to lend assistance.
"This afternoon, a group of outrageous criminals attacked our officers. Right now, we have seven officers that have serious injuries, including broken bones, and one officer who is unconscious," Capt. Eric Kowalczyk told reporters.
He vowed to find the attackers and put them in jail.
Video showed police in riot gear taking cover behind an armored vehicle, as protesters pelted them with rocks.
At one point, it looked like officers used tear gas. The Baltimore Police Department said it had heard reports of protesters setting small items on fire, and footage showed a cruiser in flames.
"You're going to see tear gas. You're going to see pepper balls. We're going to use appropriate methods to ensure that we're able to preserve the safety of that community," Kowalczyk said.
There were no immediate reports of injuries among the protesters.
Earlier in the day, the Baltimore Police Department said it had received a "credible threat" that gangs were teaming up to "take out" officers.
It did not say where the information came from, nor did it say whether the threat was tied to the recent death of Freddie Gray. Gray died in police custody under circumstances that remain unclear.
His death has sparked ongoing protests in Baltimore and raised long-simmering tensions between police and residents there.
Maryland State Police have ordered an additional 40 troopers to Baltimore to join the 42 troopers already sent there Monday afternoon to assist city police.
Since last Thursday, more than 280 state troopers have provided assistance in Baltimore.
Monday's violence came the same day as Gray's funeral. The 25-year-old was arrested on April 12 and died one week later from a fatal spinal cord injury.
"I am sure that the family is concerned, and I am positive that they are against what is beginning to develop here in town," said Billy Murphy, an attorney for the Gray family.
"They don't deserve this any more than Freddie Gray deserved it," he said about the injured officers.
In spite of the violence, the Baltimore Orioles say the baseball team has consulted with Baltimore police, and plans to play Monday night's home game against the Chicago White Sox as scheduled, a source with knowledge of the situation told CNN Sports.
Over the weekend, a few protesters vandalized police cars, threw objects at officers, cursed at them and scuffled with them.