Frankfurt police said in a statement that they had to intervene "at several locations" the previous night because of fights between rival fans or because supporters "wanted to fight." The police said they stopped major altercation around the main train station.
Police say up to 1,000 visiting supporters "including fans considered to be a risk" were registered before the game.
"The rival fan groups kept looking for violent clashes, which the police prevented by quickly intervening, especially in the station area. Despite that, there were physical attacks elsewhere," the statement said, referring to an attack carried out by Frankfurt fans that left two visiting supporters unconscious. Both men had to be hospitalized due to their injuries.
Other altercations included an attack by a larger group of Frankfurt fans on West Ham supporters in a pub, in which baseball bats were used.
Frankfurt wound up beating West Ham 1-0 in the second leg and 3-1 on aggregate to set up a Europa League final matchup with Scottish club Rangers, who beat RB Leipzig in the other semi.
There were also violent clashes in France between Marseille and Feyenoord fans outside the Stade Velodrome stadium before the return leg of their Europa Conference League semifinal.
One video showed a Marseille apparently unconscious on the ground as he was helped by fellow fans, another showed two groups charging at each other shortly before kickoff just outside the stadium.
The match ended 0-0, with Feyenoord advancing to the Conference League final against Roma, victors over Premier League side Leicester City in the other half of the draw.
This latest incident in the French seaport follows violent clashes between fans of Marseille and Greek side PAOK last month.
Although Marseille's police prefecture had said Wednesday that all supporters from Feyenoord were banned from entering the city center before Thursday, many had arrived by Wednesday and were drinking in the tourist area of Vieux Port.
The prefecture said there were 20 arrests following sporadic fights around the port and its surrounding streets on Wednesday evening and later that night.
Around 3,000 Feyenoord fans had tickets for the game, but police had anticipated more than 1,000 arriving without tickets during Thursday. The game was considered high risk and the police presence was stepped up. Large groups of Feyenoord supporters were ferried to the stadium in specially designated buses.
After the violent incidents between PAOK and Marseille, where opposing sets of fans threw firecrackers at each other during the match, UEFA closed one end of the Stade Velodrome for Thursday's visit of Feyenoord. The section located behind one of the goals holds around 13,000 home fans and was located alongside the away section.
Feyenoord have one of the more active hooligan elements in Dutch soccer. Unruly supporters from Feyenoord were reportedly among those clashing heavily with Dutch police in Rotterdam during a riot against coronavirus restrictions in November.
The match began with a thick cloud of smoke hanging over the stadium as Marseille fans lit flares in a pyrotechnics display behind the goal.
There was an incident during the second half as riot police moved to form a shield in front of some Feyenoord fans after they threw flares onto the field.