Some 40 masked and helmeted hooligans burst into a refereeing seminar in an Athens suburb on Monday, the Greek soccer federation said.
"A criminal organization which has been acting undisturbed lately continues to endanger the lives and physical integrity of people involved in soccer. Another incident of violence has been added to the chain of such events," the federation said.
The federation claimed intruders both insulted the referees and threw yogurt and other objects at them, injuring one.
"The federation strongly condemns these acts of violence. It calls on the state to protect the people of soccer and to find those criminals who are seeking to intimidate and threaten their [the referees] security," added the federation.