Baltimore ? A fight broke out among inmates in a prison yard Friday, injuring 18 prisoners, three of them critically, prison officials said.

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.