A policewoman was seriously injured following an explosion at the Vasil Levski Stadium during Levski Sofia’s Bulgarian league match with city rivals CSKA.
The officer was hit by broken glass after a device exploded near a stadium entrance, police officials confirmed.
“She underwent surgery but there are many pieces of glass on her face and in the eye,” police chief commissioner Anton Zlatanov said.
“It’s not a firework, it’s a bomb.”
Zlatanov added that “more than 20 Levski fans” were detained after the 2-2- draw.
The Sofia derby has a long history of crowd violence.
Last year, three members of CSKA Sofia’s coaching staff were hit after fans threw firecrackers and stones during a league match.