The sounds of chaos bounce off the dim yellow walls. Everywhere there are prisoners wearing orange, red and khaki jumpsuits. An officer barks out orders as a thin woman tries to sleep on a hard bench in a holding cell. This is a harsh scene of daily life inside what has become the state’s largest de facto mental institution: the Cook County Jail.

About 11,000 prisoners, a mix of suspects awaiting trial and those convicted of minor crimes, are housed at the jail at any one time, which is like stuffing the population of Palos Heights into an eight-block area on Chicago’s South Side. The Cook County sheriff, Tom Dart, estimated that about 2,000 of them suffer from some form of serious mental illness, far more than at the big state-owned Elgin Mental Health Center, which has 582 beds.

Mr. Dart said the system “is so screwed up that I’ve become the largest mental health provider in the state of Illinois.” The situation is about to get worse, according to Mr. Dart and other criminal justice experts. The city plans to shut down 6 of its 12 mental health centers by the end of April, to save an estimated $2 million, potentially leaving many patients without adequate treatment — some of them likely to engage in conduct that will lead to arrests.

“It will definitely have a negative impact on jail populations,” said Mr. Dart, who noted that the number of people coming into the jail with mental health problems was already increasing. “It will have direct consequences for us in my general jail population and some of the problems I have here, because a lot of the people with these issues act out more, as you would expect, so that’s a direct consequence.”