Ten people were taken to hospital after a chemical attack outside a pub in the early hours.
The substance, believed to be ammonia, was sprayed during a huge brawl and hit the victims, which included three police officers.
During the incident a group armed with baseball bats damaged The Cricketers pub in Aylestone, a suburb of Leicester.
All 10 of the victims were checked over for chemical-related injuries at Leicester Royal Infirmary and later discharged.
Five people were arrested over the incident on Sunday morning, during which Leicestershire Fire and Rescue Service was also called out.
Two men aged 53 and 43 were held on suspicion of causing ABH, a 22-year-old man was arrested on suspicion of a public order offence and a 29-year-old woman was arrested on suspicion of criminal damage.
A boy, 15, was also held on suspicion of possessing an offensive weapon.
All five remain in custody.
A statement from Leicestershire Police said: "Police were called to a pub near the cricket ground shortly after midnight, following a report that a fight had broken out among a group of people – some of who were carrying baseball bats.
"When officers arrived, along with colleagues from the East Midlands Ambulance Service, it was reported that some people may have been sprayed with a chemical – believed to be ammonia. As a precaution, Leicestershire Fire and Rescue Service were also called to the scene.
"Ten people – seven members of the public and three police officers – were taken to the Leicester Royal Infirmary to be checked over for chemical-related injuries. They have all since been discharged."
Enquiries are ongoing and anyone with information is asked to call Leicestershire Police on 101, quoting incident 4 of July 7.
