Fifteen people have been held by police in connection with an attack on the Bulgarian Football Union (BFU) headquarters on Saturday, the country's interior ministry said on Sunday.
CSKA Sofia fans threw stones and fireworks at the BFU building in central Sofia, smashing windows and hitting a reporter who tried to take a photograph [ID: nL3N0F508O].
"Fifteen people have been detained so far," the interior ministry's chief secretary Svetlozar Lazarov told reporters.
"We're still conducting questionings that will determine if any will be charged," he said, adding that further arrests were planned.
Police had stopped fans trying to set fire to the building, he said.
"The police prevented more serious incidents. When they arrived, some of the fans were inside the building, lighting torches and trying to damage the furniture and equipment in the building."
Supporters of CSKA, who are the most successful Bulgarian club with 31 league titles, have accused the BFU of being too strict and not allowing a merger with another club.
Debt-ridden CSKA will play in the amateur championship next season if they fail to merge with another first division club in time for the start of the league on July 20.
CSKA condemned the violence, saying in a statement: "We are sure that the millions of CSKA fans in Bulgaria do not admire such behaviour; CSKA has proved it can build, not destroy.
"We categorically state that the management of the club is not involved even indirectly in the vandalism against the building of the Bulgarian Football Union."