Last updated on .From the section European Football

The Bulgaria national team play their home games at the Vasil Levski Stadium

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.

In 2000, a man was killed by a bomb after the game and in 2014, then CSKA coach Stoycho Mladenov said he was knocked unconscious by a snowball thrown from Levski fans.

Last year, three members of CSKA Sofia's coaching staff were hit after fans threw firecrackers and stones during a league match.