Thursday evenings' matches in the Europa League were overshadowed by violence as arrests and injuries occurred in several of the venues, dpa reported
In Naples, where Liverpool and the home side played to a goalless draw ten Liverpool fans were injured during attacks and three Naples fans were arrested.
Naples mayor Rosa Russo Iervolino apologized for the attacks. "This is a blot on our page," she said, while Liverpool coach Roy Hodgson said he was pleased that nothing worse had happened.
Football fans in Sofia also rioted after CSKA Sofia's 2-0 defeat against Rapid Vienna. Four Austrian fans and four policemen where injured.
Bulgarian fans threw flares, fire crackers and bottles and also damaged two police cars. Police sources said that they had arrested 18 hooligans.
CSKA has not managed to score a single goal or secure a single point in their Europa League campaign and Bulgarian media reported on Friday that coach Gjore Jovanovski and sporting director Emil Kostadinov were fired immediately after the game.
Violence was also seen in Russia's second-largest city, St Petersburg after the home side's 2-0 victory against Hajduk Split.
Newspapers said that a group of about 50 St Petersburg hooligans wearing balaclavas had attacked traveling Hajduk fans in their hotel, injuring four in the process.
One of the victims is said to have suffered a broken nose, while the others received a concussion. It is believed that right-wing St Petersburg fans were responsible for the attack.