A retired chief inspector says someone "could've died" when police used batons on football fans after a derby game.
Opposing Wolves and Birmingham City fans clashed near Wolverhampton railway station after the home side's 2-1 defeat, West Midlands Police said.
It said officers were "forced to quell" disorder and used "appropriate" force.
But Nick Glynn, who trained police in the use of force, said he filmed "potentially unlawful violence" by officers on Friday night.
Wolves supporters threw bottles, seats and coins following Birmingham City's second goal, according to West Midlands Police.
The force said an investigation into the two incidents had been launched.
Thirteen arrests were made on the night - eight men were believed to be Blues supporters and five were believed to be Wolves fans.
Birmingham fan Mr Glynn, who served with Leicestershire Police for 30 years, said Blues fans were singing outside the railway station but he was not aware of Wolves fans being present.
He also said it "didn't sound" like public disorder.
Police were trying to force fans into the station, shouting commands to move and "shouted a show of strength command, which means that they raise their batons in the air," he said.
He stated: "[An officer] just bashed down with his baton on somebody, the head or their shoulder, that's what it's going to hit, and that was then followed by a load of other officers appearing to do the same."
Mr Glynn said: "The threat level that you would need to justify hitting somebody over the head with a baton is really high.
"Somebody could've died from getting struck on the head with a baton and, my question would be, for what? For maybe not walking quite quickly enough to a railway station?'"
Match commander Ch Insp Nick Rowe said his officers issued several warnings "to no avail" and used "proportionate force".
He said: "My officers were faced with a very challenging, yet isolated incident, and officers had to take action to prevent further clashes and other fans getting hurt.
"If we receive any complaints then this will be reviewed by the West Midlands Police professional standards department in consultation with the IPCC [Independent Police Complaints Commission]."
Ch Insp Rowe said police would look to identify those responsible for throwing missiles inside the stadium and those involved in trouble near to the station.