St. Louis police used pepper spray to disperse a crowd protesting outside the city's medium-security jail because of lack of air conditioning at the facility.
The St. Louis Post-Dispatch reports critics have sought to close the jail for years, alleging unsanitary conditions and abuse by guards.
About 150 people protested Friday evening outside the prison, demanding that the workhouse be closed. The pepper spray was used shortly after officers ordered the crowd to leave.
Temperatures were forecast to be near 100 during the weekend.
City Corrections Commissioner Dale Glass says a heat safety plan is in place, with the cafeteria and other common areas serving as cooling stations. He says medical help is available and inmates have 24-hour access to cooling items such as water and ice.