A “large-scale” fight broke out when a gang of youths confronted racegoers following Ladies Day at Aintree.
Twitter users condemned the violence describing those involved as “feral rats” and “scum of the earth”.
A 16-year-old boy was arrested over the disturbance on Copy Lane, about a mile from the racecourse.
Merseyside Police had earlier praised racegoers for their behaviour as thousands enjoyed the second day of the Grand National Festival.
Supt Dave Charnock said: “This kind of violent disorder in a public place will not be tolerated and we are currently examining CCTV and video footage and following a number of lines of inquiry to identify those responsible.
“Any groups of people gathering in the areas of Old Roan to Warbreck Moor can be moved on and I would ask anyone who witnessed the incident or has any footage to contact us.”
The suspect is being held on suspicion of affray, the force said.
Two arrests were earlier made at the course, with one man held on suspicion of possession of counterfeit currency and another arrested on suspicion of a public order offence.