A policewoman has undergone surgery on an eye injury after being injured in an explosion during a derby between Bulgarian rivals Levski Sofia and CSKA.
Police said more than 20 fans were arrested at the Vasil Levski stadium during Levski's match against their city rivals on Wednesday.
Reuters reported police chief commissioner Anton Zlatanov as saying a bomb had been thrown from the Levski fans' section, also injuring another police officer.
Zlatanov said the officer who had needed treatment was "still in bad condition."
"She underwent surgery but there are many pieces of glass on her face and in the eye," he said.
"Such an act is inadmissible. It's not a firework or a small bomb, it's a bomb. This is an extremely unpleasant and ugly incident."
The derby, which finished in a 2-2 draw, has a history of crowd problems, with a man having died in a blast in 2000 and violence and arrests common.
