Authorities said that no employees were hurt in the brawl at the Metropolitan Transition Center in east Baltimore, but that the prison was being locked down.
Details about the cause of the disturbance were not immediately available, but it involved several groups of inmates, some of whom were armed with homemade weapons, corrections spokeswoman Major Priscilla Doggett said.
Emergency crews found inmates with injuries ranging from lacerations to stab wounds, Baltimore Fire Chief Kevin Cartwright said.
"Our paramedics arrived here in quite a chaotic situation," he said. Paramedics had to wait for correction officers to secure the yard before they could treat the injured, he said.
The entire facility will be searched for contraband, Doggett said. All inmates were returned to their housing area by Friday afternoon.
The facility will remain on indefinite lockdown, and the emergency operations command center has been activated, she said.
The transition center, a minimum-security facility, houses more than 1,700 inmates with two years or less to serve on their sentences. It is in part of the former Maryland Penitentiary, an imposing, gothic structure built in 1811 that was once a maximum-security prison.