BALTIMORE, M.D. (CNN) - Seven Baltimore police officers have suffered major injuries, including one who is unconscious and others with broken bones during clashes with protesters Monday, Capt. Eric Kowalczyk said.
He described those responsible for the violence as "outrageous criminals."
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Officers are working to restore order and arrest those who attacked police, Kowalczyk told reporters.
"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," he said.
Video showed police in riot gear taking cover behind an armored vehicle, as protesters pelted them with rocks.
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 Gray's death. 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.
"The Baltimore Police Department/Criminal Intelligence Unit has received credible information that members of various gangs including the Black Guerilla Family, Bloods, and Crips have entered into a partnership to 'take out' law enforcement officers," police said.
"This is a credible threat. Law enforcement agencies should take appropriate precautions to ensure the safety of their officers," the statement read.
Over the weekend, a few protesters vandalized police cars, threw objects at officers, cursed at them and scuffled with them.
About a dozen young men smashed squad cars with garbage cans, climbed on top of them and stomped on them, CNN video showed.
Also on Monday, hundreds attended Gray's funeral. The 25-year-old was arrested on April 12, and died one week later after suffering a fatal spinal chord injury.