Frankfurt am Main police said two West Ham supporters had been knocked unconscious and were taken to hospital after being attacked by apparent Eintracht fans.
That incident occurred early on Wednesday evening in the Schulstrasse area of the city centre, ahead of Thursday's match.
A police statement issued on Thursday confirmed: "Here, a group of violent home fans attacked a group of away fans, knocking two of them unconscious. Both men were taken to a hospital with injuries and had to be hospitalised."
The police reported "a larger group of violent criminals", who were thought to be Eintracht supporters, targeted West Ham fans later in the evening on Taubenstrasse, with that baseball bat attack causing damage at a restaurant and leading to a bar worker suffering "minor injuries".
The police added: "In the evening hours, the crowd of English fans was concentrated on Munchener Strasse. An estimated 800 guest fans were to be found here, including around 150 risk fans. Due to the accumulation, Munchner Strasse had to be closed to both road and rail traffic. Large processions of WHU [West Ham United] supporters formed twice in the evening and at night on Munchener Strasse to march through the station district. The police prevented this both times with timely and consistent intervention.
"At around 11:30pm, supporters of both camps sought an argument on Gutleutstrasse. The emergency services also stopped the imminent confrontation here. The police arrested 15 violent supporters of the home team."
Police reported the trouble began to abate afterwards, and the street was reopened to traffic. "In total, the police arrested more than 30 people yesterday," they said.