(CNN) -- Chaos and violence erupted on the streets of Baltimore late Monday as protesters clashed with police, several of whom were injured.
Maryland Gov. Larry Hogan declared a state of emergency and activated the National Guard. The mayor of Baltimore said every possible resource was being deployed to "gain control of this situation."
"What we see tonight that is going on in our city is very disturbing," Mayor Stephanie Rawlings-Blake told reporters. "Too many people have spent generations building up this city for it to be destroyed by thugs who -- in a very senseless way -- are trying to tear down what so many have fought for."
She announced that the city will impose a mandatory curfew from 10 p.m. to 5 a.m. daily, effective for one week starting Tuesday. The mayor stressed that the city already has a mandatory curfew for young people -- 9 p.m. ET for children under 14, while teenagers 14-16 have to be inside by 10 p.m. ET on school nights.
Rawlings-Blake also promised to hold those responsible for the violence accountable.
More than two dozen people have already been arrested, according to Col. Darryl D. DeSousa with the Baltimore Police Department.
He said that 15 officers were injured. Two remain hospitalized, and the others have been released. Most were hurt by flying debris in the violence that DeSousa described as "unprecedented."
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.
Video also showed people looting local stores, and an area CVS pharmacy was set on fire after it was ransacked. Thick dark smoke filled the streets.
There were no immediate reports of injuries among the rioters.
"It is so frustrating that people think that this makes sense -- to destroy our community," said Rawlings-Blake. "People who live there, that are already hurting, are going to be the ones that pay."
'Credible threat' to officers
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.
"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."
Up to 5,000 law enforcement officials will be requested from the mid-Atlantic region to help quell the violence in Baltimore, Col. William Pallozzi of the Maryland State Police said at a press conference late Monday. Also, authorities say that about 1,500 members of the National Guard have been deployed.
Maryland State Police 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.
"Today's looting and acts of violence in Baltimore will not be tolerated," said Gov. Hogan. "There is a significant difference between protesting and violence, and those committing these acts will be prosecuted under the fullest extent of the law."
Separately, President Barack Obama met with Attorney General Loretta Lynch at the White House, just hours after she was sworn in.
In a statement, she condemned the "senseless acts of violence" in Baltimore, and said the Justice Department "stands ready to provide any assistance that might be helpful."
"Those who commit violent actions, ostensibly in protest of the death of Freddie Gray, do a disservice to his family, to his loved ones, and to legitimate peaceful protesters who are working to improve their community for all its residents," said Lynch, urging residents of Baltimore to be nonviolent.
'They don't deserve this'
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.
Because of the violence, the Baltimore Orioles have postponed their home game against the Chicago White Sox on Monday, the team announced via Twitter. At first, the team planned to go ahead with the game, but delayed it after further talks with the Police Department. A new date for the game will be announced later.
Baltimore City Public Schools will be closed Tuesday, according to schools spokeswoman Edie House-Foster.
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.
"This has got to the point, this is not about Gray right now. It's reactionary," a Baltimore resident and protester said on Monday. "You can only put so much into a pressure cooker before it pop."