Fan trouble has kicked off both before and during the Europa League match between Antwerp and Eintracht Frankfurt in the Belgian city, while Marseille versus Galatasaray was interrupted for about 10 minutes after rival fans threw flares and firecrackers at each other.
At Antwerp on Thursday, visiting keeper Kevin Trapp received treatment after a pyrotechnic device thrown from the crowd exploded near his feet, while angry home fans tried to storm the pitch after Goncalo Paciencia had slotted in a 91st minute spot-kick winner in a 1-0 victory for Frankfurt.
The Belga news agency had reported that, earlier, visiting fans attacked an Antwerp fan bar and destroyed its terrace before the match.
Police had been on high alert throughout the day and have made several arrests.
"This does not belong in football," said Frankfurt coach Oliver Glasner, referring to the flare being thrown.
In the same competition at Marseille's Stade Velodrome, despite an increased security presence and a buffer zone between around 2,500 visiting fans from Turkey and the home supporters, objects were thrown in both directions late in the first half.
Referee Pawel Raczkowski halted the match as some police then intervened to restore order.
Galatasaray coach Fatih Terim went over to his side's fans to try and calm things down, while Marseille captain Dimitri Payet did the same.
The match resumed with about eight minutes left in the first half with the sound of firecrackers still going off.
Marseille were recently involved in fan disorder during a French league game at Angers.
In Prague, Rangers midfielder Glen Kamara was repeatedly booed by the thousands of young Sparta Prague supporters.
Normal fans were banned at Sparta's stadium for the Czech club's 1-0 victory because of previous racist behaviour, but UEFA allowed the hosts to have the support of some 10,000 schoolchildren aged 6-14.
The booing of Kamara came after Ondrej Kudela from Sparta's arch-rival, Slavia Prague, received a 10-match ban from UEFA for racially abusing the Finnish player in a knockout game in the Europa League last season.
Kamara was sent off Thursday after picking up his second yellow in the second half.
The ban from all UEFA competitions prevented the 34-year-old Kudela, who was a starter for the Czech Republic, from playing at this year's European Championship.
Sparta was ordered by UEFA to play a game without fans due to racist abuse of a player during a Champions League qualifier at home to Monaco on August 3.
A group of fans directed monkey chants at Monaco midfielder Aurelien Tchouameni after he scored the opening goal in the first half, and again when the game was over.