More than 20 Levski Sofia fans were detained after a policewoman was seriously injured following an explosion at the Vasil Levski stadium during the 26-times Bulgarian champions' league match against bitter city rivals CSKA on Wednesday.

Both the female officer and a male colleague were hit by broken glass from a window that smashed when the device exploded close to the stadium entrance, said officials from the ministry for the interior.

"She is still in bad condition," police chief commissioner Anton Zlatanov said. "She underwent surgery but there are many pieces of glass on her face and in the eye."

"Their injuries would have been much worse had they not been standing behind the glass door," he added.

"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."

According to Bulgarian media reports the female officer risked losing an eye. She underwent surgery for damage to her face.

He said the bomb had been thrown from the Levski fans' section at a door near one of the stadium entrances, adding that another police officer sustained minor injuries.

"More than 20 Levski fans were detained but I can't tell the exact number," Zlatanov said.

The Eternal Derby, as matches between the two most popular clubs in the Balkan country are called, ended in a 2-2 draw. CSKA are second in the standings, eight points behind leaders Ludogorets. Levski are fourth.

The fixture has a long history of crowd violence.

A man was killed by a bomb after the game in 2000 and hooliganism and arrests have been rife since then.

In addition to the rivalry on the field, the two teams also have an ongoing political feud, with several Levski fan groups having ties to the far-right and CSKA with to the far-left.