One policeman and three football supporters were injured and eight people were arrested in clashes between about 300 football fans in downtown Sofia, the interior ministry announced.
About 300 supporters of football arch rivals CSKA and Levski were involved in the fight, some armed with batons and metal rods, the ministry said in a statement.
Three fans, aged 14, 17 and 28, were taken to hospital after the fight but were in a stable condition while one policeman sustained an arm fracture, the ministry said.
Media reports said one of the supporters had been stabbed in the face with a ski pole, while another had knife cuts.
Six fans of Levski and two of CSKA were arrested, the ministry said, adding it would likely make more arrests.
Media reports said CSKA fans had gathered to attend a basketball game at the opening of a new hall at their Bulgarian army home ground when they were attacked. Other reports said the fight was organised via social media.
Levski and CSKA are Bulgaria's most popular teams and traditional arch-rivals and their fans frequently clash.
Bulgaria's football league is now in its mid-season break with matches resuming on March 2.
Levski are currently second in the table with CSKA sitting fourth.