SOFIA Bulgarian police on Wednesday arrested 21 football fans in connection with crowd violence this season, three days before the highly-charged Sofia derby.
"Detectives had spent recent weeks examining evidence, including pictures and video footage of the trouble," police said in a statement, adding further arrests were planned.
Police said the trouble-makers had been arrested for violence at eight domestic league games and Bulgaria national team matches, all of which took place in Sofia.
CSKA Sofia play bitter city rivals Levski in the 140th "eternal derby" on Saturday with organisers expecting a big crowd after Bulgaria's decent run in the World Cup qualifying campaign helped spark enthusiasm from fans.
Fan violence in the stands and outside the stadium marred the "eternal derbies" in the years after Communist rule was overthrown in Bulgaria in 1989 with police unveiling strict security measures ahead of every game recently.
Last year, Sofia municipality announced a series of measures to combat football violence, including early kickoff for matches between Levski and CSKA, which should be played only at the Vasil Levski national stadium, situated in the city centre.
(Reporting by Angel Krasimirov, editing by Justin Palmer)