The invention relates to an extruded solid catalyst body for breaking down nitrogen oxides in the presence of a reducing agent as well as to a process for manufacture of said body. The solid catalyst body has an active material that contains 60 to 87% by weight of an ion-exchanged zeolite containing at least one metal from the group containing Cu, Hf, La, Au, In, V, lanthanoids and transition metals of group VIII of the periodic system, more than 10 to 37% by weight of aluminum oxide and 2 to 10% by weight of inorganic fibers. The solid catalyst body, the active material of which contains a zeolite can be manufactured by extrusion and has a high degree of mechanical stability at a high catalytic activity.